Q: iTunes 12.3 can't add album artwork

I subscribe to iTunes Match

Did it work before you subscribed?

These instructions will have to be modified if you've moved the iTunes folder from its usual location.

Back up all data before proceeding.

Triple-click anywhere in the line below on this page to select it:

~/Music/iTunes/Album Artwork/Cache

Right-click or control-click the line and select

          Services ▹ Reveal in Finder (or just Reveal)

from the contextual menu.* A folder should open with a subfolder named "Cache" selected. Quit iTunes if it's running. Move the selected folder to the Trash. Relaunch the application and re-download all album artwork.

*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination  command-C. In the Finder, select

          Go ▹ Go to Folder...

from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.

I don't use iTunes Match, but from what I can gather this is a common problem. Here is the official line:

Resolve issues with iTunes Match - Apple Support

Linked below are several threads in which you might find ideas for a solution. I have no way of testing any of them.

iTunes Match - Completing Album Artwork?

iTunes Match and album covers

iTunes Match artwork reverting to old version