Ziggy

The issue is this value that doesn't allow the section to respond to the contents which is why min-height is generally used. If you want a maximum height you will need to ensure that your content is much smaller, and potentially using a responsive value like vw instead of px  for the text.

This is the message I see in place of the code, looks like a problem with the host not your website:

If you want to place this in Header Code Injection, it would need to be presented like this:
<style> .sqs-block-button-element-medium { border-width: 10px !important; } .product-quantity-input { display: none !important; } .product-quantity-input { display: none !important; } .sqs-block-button-element-medium { border-width: 10px !important; } .sqs-add-to-cart-button-wrapper, .product-quantity-input { display:none; visibility:hidden; } </style> Without the style tags, it gets rendered as plain text and will not have any effect on the page styling.

Here's your code with some corrections, a major one being this text which is not appropriate to just be dropped in to custom css:
 https://static1.squarespace.com/static/61a3588efb96aa035e93a9d2/t/61b4bacdee5d05012603bfa7/1639234253725/OPTIBakerDanmarkOne.otfhttps://static1.squarespace.com/static/61a3588efb96aa035e93a9d2/t/61b4bacdee5d05012603bfa7/1639234253725/OPTIBakerDanmarkOne.otfhttps://static1.squarespace.com/static/61a3588efb96aa035e93a9d2/t/61b4bacdee5d05012603bfa7/1639234253725/OPTIBakerDanmarkOne.otfhttps://static1.squarespace.com/static/61a3588efb96aa035e93a9d2/t/61b4bacdee5d05012603bfa7/1639234253725/OPTIBakerDanmarkOne.otfhttps://static1.squarespace.com/static/61a3588efb96aa035e93a9d2/t/61b4bacdee5d05012603bfa7/1639234253725/OPTIBakerDanmarkOne.otf
There were also missing closing curly brackets and misplaced brackets.
@font-face { font-family: 'OPTIBakerDanmarkOne'; src: url('https://static1.squarespace.com/static/61a3588efb96aa035e93a9d2/t/61b4bacdee5d05012603bfa7/1639234253725/OPTIBakerDanmarkOne.otf');} custom { font-family: 'OPTIBakerDanmarkOne'; } h1 { font-family: 'OPTIBakerDanmarkOne'; } h2 { font-family: 'OPTIBakerDanmarkOne'; } h3 { font-family: 'OPTIBakerDanmarkOne'; } h4 { font-family: 'OPTIBakerDanmarkOne'; } .collection-type-products .grid-title { font-family: 'OPTIBakerDanmarkOne'; } /* Product title font */ h1.ProductList-title, h1.ProductItem-details-title { font-family: 'OPTIBakerDanmarkOne' !important; } @media only screen and (min-width: 641px) { #collection-61b4d4bad005f33f5c742c0c { #page .page-section:nth-of-type(2) { /*second second on the left*/ width: 45%; min-height: 80vh; align-items: center; float: right; } } } @media only screen and (min-width: 641px) { #collection-61e3126680afe51b5a5aaec2 { #page .page-section:nth-of-type(2) { /*second second on the left*/ width: 45%; min-height: 80vh; align-items: center; float: right; } #page .page-section:nth-of-type(3) { /*third section on the right*/ width: 55%; min-height: 80vh; align-items: center; } #page .page-section:nth-of-type(4) { /* forth section full width again */ clear: both; } } } @media only screen and (min-width: 641px) { #collection-61f06508f54d9138d8d1fff4 { #page .page-section:nth-of-type(2) { /*second second on the left*/ width: 45%; min-height: 80vh; align-items: center; float: right; } #page .page-section:nth-of-type(3) { /*third section on the right*/ width: 55%; min-height: 80vh; align-items: center; } #page .page-section:nth-of-type(4) { /* forth section full width again */ clear: both; } } } @media only screen and (min-width: 641px) { #collection-61b4d4bad005f33f5c742c0c { #page .page-section:nth-of-type(5) { /*second second on the left*/ width: 45%; min-height: 80vh; align-items: center; float: right; } #page .page-section:nth-of-type(6) { /*third section on the right*/ width: 55%; min-height: 80vh; align-items: center; } #page .page-section:nth-of-type(5) { /* forth section full width again */ clear: both; } } } @media only screen and (min-width: 641px) { #collection-61e3126680afe51b5a5aaec2 { #page .page-section:nth-of-type(5) { /*second second on the left*/ width: 45%; min-height: 80vh; align-items: center; float: right; } #page .page-section:nth-of-type(6) { /*third section on the right*/ width: 55%; min-height: 80vh; align-items: center; } #page .page-section:nth-of-type(4) { /* forth section full width again */ clear: both; } } } @media only screen and (min-width: 641px) { #collection-61a36d3b3ed02a77d10ca846 { #page .page-section:nth-of-type(2) { /*second second on the left*/ width: 50%; min-height: 80vh; align-items: center; float: right; } #page .page-section:nth-of-type(3) { /*third section on the right*/ width: 50%; min-height: 80vh; align-items: center; } #page .page-section:nth-of-type(4) { /* forth section full width again */ clear: both; } } } /* Product details page logo */ body.collection-type-product.view-item .Header-branding-logo { filter: invert(1); -webkit-filter: invert(1); } #cart .header-title-logo img { filter: brightness(0) invert(0); } body.collection-type-products.view-item .header-announcement-bar-wrapper { background-color: #D9E2F7 !important; }  

I'm not sure exactly where it coming from but this CSS (add to the page header in style tags) should hide the element that is causing the white space:
#st-el-1.st-hidden { display: none; }

Use this in Custom CSS:
#collection-662706eb09ae990a9dd9f175 { #header { display:none; } .color-overlay { display: none; } }  

The navigation is transparent, but not the clicking.
Try this:
.header-announcement-bar-wrapper { pointer-events:none; } .header-title-logo, .header-layout-nav-right .header-nav-list { pointer-events: all !important; } It might break the header usability, but worth a try!

Try this in Custom CSS:
@media only screen and (max-width:640px) { #block-yui_3_17_2_1_1713730469225_14781 { max-width:140px; } }  

Looks like it's all for a timer and animated counter. You can always copy it, back it up, and then remove it to see what happens, if it's necessary code you can always replace it.

You can make it more difficult, but there is no way of preventing someone copying your content if you make it publically accessible, if you can see it you can copy it. Have a read of this thread which offers some methods to make it a little more difficult:

It does. Look in Image Effects the editing the section background.
 

Where did you get the code? 
Add this to the lock page code injection:
<style> *:focus { outline: none !important; } </style>  

Whilst logged in to your account and website, go to Settings -> Domains and you can change the domain there and/or link a new domain etc.

Nope, but there are countless QR generators online that you can use should you wish.

Follow this to improve the design:
 
Or add this to Custom CSS to hide the slideshow on mobile:
@media only screen and (max-width:767px) { section[data-section-id="661565bbb54e6854c3a04ba2"] { display:none; } }  

Haha, that's not right!
Try this:
#collection-655b50b045a13147527cba9b { section[data-section-id="6614f78973b88f33805c6976"] { :not(.has-background) .section-background { background-color: #e1dfd9 !important; } } }  

Try this in Custom CSS:
section[data-section-id="6616a7ba048a76656b6e95a7"] { position : -webkit-sticky !important; position : sticky !important; top : 0px !important; z-index : 99999; } Let me know if that works.

Add this to Custom CSS
html { scroll-behavior: smooth; }  

Might be a silly question, but does this work if you put it in a code block on the page rather than in the header code injection?

Instead of using a RETURN (adding a paragraph break) try SHIFT + RETURN to add a line break.

Try adding this inside the last curly bracket:
.products.collection-content-wrapper .grid-meta-wrapper { border-top: 1px solid #000; padding: 12px 16px; }  

The website is Squarespace, but the popup is not, they are using Unbounce for that:
https://unbounce.com/product/popups-and-sticky-bars/
There isn't a way to add more fields in the Squarespace popup hence utilising this system, but there are many other 3rd party systems you could use.

This should work:
.header-menu-nav-item:nth-child(2) { display : none; }  

Have you followed all of these steps?
https://support.corepractice.com.au/hc/en-us/articles/115005453608-How-to-embed-our-Online-Bookings-on-your-site
Including "whitelisting" your domain? (Squarespace and custom domain?)
https://support.corepractice.com.au/hc/en-us/articles/4412936827407

This plugin does what you're asking for:
https://www.will-myers.com/products/p/sliding-image-banner