Sticky Product Categories

Site URL: https://www.furnish-link.com/3dmodels

I would like to make the product category navigation at the top of my site sticky when I scroll down the page but I cant really seem to figure it out. I have found this code online but it does not seem to work:
 

.lessons .nested-category-children
{ position:sticky!important;
top:0; z-index:99!important;
background:#FFF!important }

Please, any help would be greatly appreciated!

Page URL: https://www.furnish-link.com/3dmodels