Also when you do import the MP3, where exactly is that file located (please right-click or CONTROL-click the file > Get Info and look at “where”).
Is the file in a folder in the Dock? Does its name or the name of any folders it is in contain symbols or accented (non-English) characters?
How exactly are you launching Audacity when you try to launch it without opening the file? Where is Audacity located?
Thanks for your patience.
Gale