There have been some issues reported with iOS15. Not sure if that is one of them but search here for those posts. I’m on Android so no experience with that OS.
I have experienced the issue previously but this was before iOS 15. For the life of me, I can’t remember what I did to resolve the issue. It did take several minutes before I got the verify button to work.
Some things to try:
type in the code instead of copy/paste
try using cellular only or wifi only
my favorite, turn off bold text in settings > display and brightness
For some reason, in my foggy memory, I believe it was typing in the code manually that eventually worked instead of clicking on the code from SMS and letting it get pasted into the field by iOS.