Steve-
No matter where I moved the file to, it was unable to open.
I think I have solved the issue, though.
It seems that Windows Defender was blocking Audacity from creating the temporary files necessary to open and edit an .aup3 file.
Disabling Windows Defender has resolved my issue.
Perhaps I am the only one encountering that problem, but if not, hopefully that helps someone else.
Thank you for your help!
Thanks for the update.
That’s quite surprising as Windows Defender is enabled by default, but clearly “most” users don’t see this problem 
Pleased to hear you’ve got it working.
Windows Defender
i’m not on windows since few years but windows defender is shitty i’m not surprise about this problem
Did you have “Controlled Folders” enabled in Windows Defender? (I think it’s off by default)
If so, then it’s probably that feature that is blocking Audacity. When enabled, that feature will block all apps that it does not recognise from writing to disk.
That’s a rather harsh assessment. Most professional reviews rate Microsoft Defender as “average” for security and “good” for being lightweight.