How to set custom GarageBand loop to tempo?

I've been playing around with GarageBand for awhile now, but I've never figured out how to make a tempo-based custom loop. When I have the part that I want as a loop, I go to add to loop library, but once the box pops up I don't have the option to select Loop. It's just grayed out

Let me know if my problem is unclear


******Hey everyone. For anyone else that might ever have this problem, I figured out what was going on. When I clipped the song in GarageBand, it ended between two beats. You have to click up at the top to make sure it ends on an exact beat or else it would throw a tempo off if it were saved as a tempo loop. Hope this helps someone some time.*****