Hey there, Erik.
We understand that you moved your music library from one computer to your MacBook Air, where the m4a files are missing and you're unable to add drag and drop music into a playlist.
Check out the information below from the following article: Import music from a computer to Music on Mac
"By default, Music places a copy of each audio file in the Music folder (Home/Music/Music) and leaves the original file in the current location. For information about adding an alias to the file (instead of creating a copy), see Change where your music files are stored.
When you import an item by dragging it to the Music window or choosing File > Add To Library or File > Import, a reference (or “pointer”) to the item—not the item itself—is placed in the Music folder. You might not be able to play the song if you move it."
Have you had the chance to search there for your missing songs?
Also, you can quit the Music app to see if that helps with the playlist issue: Quit apps on Mac Another step that may also help is to restart: Log out, sleep, restart, or shut down your Mac
If you'd like, you can read more about playlists here: Intro to playlists in Music on Mac
Let us know if that helps or if you have any questions. Take care!