This misleading error message can occur on some damaged .aup3 projects as the open and import logic is somewhat circuitous.
So it is likely that this particular .aup3 file is trashed. See cannot import AUP3 format, Use file> open instead and cannot import aup3 format. use file>open instead
I suppose you could try this recovery procedure: Corrupt or Otherwise Broken Audacity Project Recovery. Good luck. Report back what you find.