Help, moving iTunes library from Mojave to Music in Monterey.

When you hold down the alt/option key as you launch Music you should get this dialog box:

If Music opens normally close it and try again.

You would click Choose Library..., select the old iTunes Library.itl file and then choose a location to save the converted library. Are you unable to get that dialog to show, or does it not process the .itl file correctly after selecting it?

If you don't mind losing your ratings, playlists, play counts, etc. then you can import the media you have with File > Add To Library... into the library you currently have but it would seem to be better to grab those details if you can. Make sure to set the media folder in Music to match the legacy location from iTunes, typically ~/Music/iTunes/iTunes Media, so Music doesn't generate new copies of the files that it imports.

tt2

You could try Repair security permissions for iTunes/Music for Mac - Apple Community in case there are permissions issues. Alternatively you might try saving the converted library file on internal storage in case Music is being fussy for reasons.

tt2

Using Music > Preferences > Files to define a new media folder tells the app where to store new ripped, downloaded, or converted music in future, it doesn't make the app scan that folder, nor will it import the metadata (playlists, ratings, etc.) from the old .itl file.

tt2

Press and hold down option as you launch Music.

Keep holding until prompted to choose or create a library.

Click Choose Library.

Browse to and open your iTunes Library.itl file.

Choose a location save the converted Music library.

tt2

It should be straightforward. I've tested the process with a new iTunes test library created in Windows copied to Catalina 10.15.7 and Monterey 12.4 and 12.5 and it worked correctly each time. Do you have any backups of your Mojave .itl file? Perhaps an older copy would work.

tt2