iTunes 12.5.4 'add artwork' greyed out? HELP

I am trying to add album artwork to the WAV files of my new album that I will be uploading to websites like bandcamp.com as well as streaming services like Apple Music.

iTunes 12.5.4.42

When I right click the song Get Info > Artwork, the "Add Artwork" is greyed out.

NOTE: I have the files privilege set to (Me) Read & Write

When I select all of the files at once it seems to allow me to upload artwork, BUT once I click 'OK' nothing saves and it does not work.

Can anyone please help? I need these WAV files correct with artwork as they will be uploaded for download online.

Thank you!