Can you make a new project and successfully open it?
Did you (or somebody) copy or move the AUP file? An AUP file is only the “master file” for an Audacity Project.
It’s always a good idea to export to WAV (whither you make an Audacity project or not) because a WAV is a single file and it’s harder to accidently mess it up (or you can make multiple WAV files depending on what you’re doing).
Both parts have to be in the same location or folder, they have to be named the same and it has to be the name you gave them inside Audacity.
Having a _DATA folder missing is pretty common and the error message usually says so.
I’m trying to play this out in my head. You might get this error if the project opens looking for a specific microphone or sound interface. If the project opens in a foreign computer, the microphone won’t be there and that might be the complaint.