Macbook refusing to transfer music to iphone

The Music app is built into macOS. You cannot delete the copy you have nor reinstalling except by reinstalling the whole OS. Reinstalling Music doesn't reset the database. You can hold down option as you launch Music to create a new library, e.g. at /Expansion/Music/. You could then move the media folder from the current location to /Expansion/Music/Media and import the contents of that folder to your library. The new library would be missing ratings, play counts, playlists, any metadata associated with .wav files, and every track would have the current date/time as its Date Added info.

These instructions aren't that complicated:

Alternatively download using this link - FixLinks.zip - and extract the file. Save to ~/Library/Music/Scripts if you want to be able to call it from the Music menu bar.

You can open FixLinks.scpt, the extracted file, by double-clicking on it in Finder. There is a play button in the Script Editor to run it. You then respond to prompts from the script.

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Storing the music library in a non-standard location has always been twitchy. Highly recommend you don't do that.