You may need to convert your file to a format that Audacity supports (Audacity's documentation gives a list of supported formats).
Install the FFmpeg extension (this is the easiest solution)
Follow the directions on Audacity's documentation here: Installing FFmpeg
OR
Converting files to mp3 format on a Mac using Music:
- Open the file in Music.
- Click on the file.
- In the Music menu bar at the top of the screen, select File > Convert > Create mp3 version.
- Let Music convert the file, then drag it onto your desktop to use.
- Open the file in Audacity and begin editing.
OR
Converting files on a PC:
- Download a free conversion program and follow its directions.
- Once the file is in a format that Audacity supports, open the file in Audacity and begin editing.