I had the same problem with my T key also - the top wouldn’t “click” in. Searched online for ages and couldn’t find an answer.
I then took off a different key cap and attached it in the place of the T and it fitted perfectly. So this narrowed it down to the key itself and not the mechanism.
As a last ditch attempt I squeezed the tiny “C” shaped clips at the top of the T key between my fingernails (these are different to the hooks at the bottom). I didn’t think it did anything, but to my surprise there was resistance when I went to put it back on, and it now works!
'''TL;DR:''' Try to squeeze the tiny clips together at the top of the key between your fingernails (or gently with some needle nose pliers maybe). The goal is to make them slightly tighter. They must slightly distort with use. It doesn’t take much, so don’t overdo it!