N9021M is taken. It's currently assigned to a 1996 Weatherly Aviation Co Inc 620B registered in Bellingham, WA.
See the full aircraft record →A tail number only frees up if the owner deregisters the aircraft or moves it to a different N-number — and even then the FAA holds it before releasing it to the public.
N9021M has been worn before. Past registrations from FAA deregistration records:
Tails one nudge away from N9021M that aren't assigned or reserved: