import songs into the Music App from a folder on my hard drive

here are many ways to add an audio file to your iTunes/Music library. Once you are sure the valid file is on your local drive, try one or more of the following:

• Drag it into iTunes (or Music)
• Drag it into a specific playlist (i.e., the open window of a regular playlist, not a smart playlist)
• Drag it into the "Automatically Add..." folder (Do this while iTunes/Music is running)
• Use the command File > Import and navigate to its location
• Right-click the file and choose Open With > iTunes (or Music)
• Double-click the file

If the audio file is valid, at least one of these should work.

If none of them work you may have an invalid or corrupt file. This has been happening a lot lately with files from "questionable" sources. If this is your situation, use a 3rd party tool to clean up or convert the file, or seek a more reliable source.