New pages won't allow spacer blocks

I've been helping a friend with her SS website, and her requirements have caused me to use multiple code blocks per page to control spacing and function.

I've built about ten pages, using the same process: 1) create blank page, 2) create blank section, 3) add blocks one at a time, peppering each page with spacers and code blocks (as well as other blocks).

For some reason, it's now in a state where whenever I create a new blank page, and add a blank section, It shows a grid of boxes (see first attached image), and the block adding options no longer include "spacer" (see second image).

I don't know what's different between the old pages (no grid, spacer available) and the new pages (grid, no spacer), but it's limiting my ability to tune the UI.

What am I missing?

Welcome to the Fluid Engine! It's Squarespace's brand new editor, I would recommend that you read up about it here first: 
https://support.squarespace.com/hc/en-us/articles/6421525446541-Fluid-Engine-Squarespace-s-latest-content-editing-experience
And then browse around on the forum to see how people are using it:

Fluid Engine (FE) has been a little controversial as it changes the workflow of building a section, it has more possibilities as you can overlap blocks and design a different layout for mobile, but there are still some issues with responsive spacing and it is a more lengthy design process as you now have two layouts for each section that need building and managing (desktop and mobile).

You can still add "Classic Sections" if you're a Circle Member, and still disable the Fluid Engine Editor on new websites if that's what you want to do.

(There is no spacer block on FE as they are no longer necessary, you can just leave a blank row)