I’m trying to transfer my music library to my new computer

The fastest and easiest way, is to Migrare from your Time Machine backup when you first setup the new iMac.

see > https://support.apple.com/en-us/HT204350

If you do not have a Time Machine backup, then you will want to Migrate using an Ethernet connection between the two Mac’s. Do not Migrate over you local Wi-Fi network, because it is prone to failure.

Please refer to Setup Assistant and Migration Assistant to learn the easiest and most efficient method of migrating your data.

You can check this link.

Move your content to a new Mac - Apple Support