When you hover it you will see why it’s disabled.

Achieved this by enabling the “Clip content” of the frame.

thanks @Gleb

I want to make an outline for a group of several shapes. I know I cannot use stroke or set drop shadows for the individual shapes, because they overlap and the outline of one shape ends up on top of another shape. I thought an easy solution would be making a spreaded, non-blured drop shadow. How do I get it to enable? It says “only supported for frames or components with visible fill paints”. Thanks!