Button set option not available?

Hello,

I'm attempting to create a flashcard set, and my desired outcome is that when a learner clicks on a flashcard, a box expands and within that box they are able to tab between three columns of content. 

I am planning to create a layer for each of the six flashcards. While trying to build within the first layer, I am not able to create a button set out of the shapes labeled DB 1, 2, and 3. The second file I'm attaching is the inspiration I had for this flashcard set (can also view here). 

Can anyone provide insight on why the create button set option is not available? Is it not possible to create a button set within a layer? 

Thanks in advance!