Hello, I am having the same issue. I am using iOS 14.5 and there is always authorization status "RESTRICTED" when requesting ATT permission.
Tracking setting is auto-disabled and not allowed to change.
My account do not appear to be in any sort of restricted state (age, MDM, EDU).
On my other device (with iOS 14.4) with same Apple ID I can see the prompt and ATT setting works fine.
Device reset (with no data restore) didn't help.
(My Feedback assistant report - FB9089120)