How to use Rollup to count instances in a relational column?

Hey, I’m using Glide App to create a material lists organized into categories.
My goal is to count the number of materials in each category. This is a relational column that looks at another table for a category to match to all materials.
Why this doesn’t work?

If you want the info (rollup) from the relation, you need to select “Category?Materials” them one of the two columns either title or description.

Adding to this, then use a “Count” method to return the correct instances.