We need to try and diagnose what the actual problem is.
We now know that the problem is NOT: Audacity will not import any file.
The problem IS: Audacity will not import “some” files.
The next questions should be:
What exactly happens when Audacity attempts to import one of the problem files?
Are there any error messages?
Does it import at all? (for example, importing as a flat line or importing as loud noise, or as an empty track, or…)
Does anything else happen? (for example, does Audacity crash, or freeze)
What do the problem files have in common?
Where did they come from?
Do they play in other apps?
What is the exact format?
Can we verify that they are properly formed?
Are they all very big files, or very small files?
What does “MediaInfo” say about the files? (Test the files here: https://mediaarea.net/MediaInfoOnline)
Do you have a small “problem file” that you could share with us?