Sync music from my computer while iCloud Music Library is on?

Tracks that you add to your iCloud Music Library should be matched or uploaded to the cloud so that they become available to each of your devices to stream or download without needing to be synced from your computer. See Identify cloud status icons in your music library on your Mac or PC - Apple Support to see if there are issues with your imports that need to be resolved. If you have tracks that show waiting, error, etc. select them, right-click and try using Add to Cloud Library. Then try File > Library > Update Cloud Library.

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See Authorization vs. association - Apple Community. Your iCloud Music Library can only contain purchased content from your primary Apple ID. If you made purchases from multiple IDs in the past then a workaround to enable your older content would be to convert the files from 256K ACC to 256K ACC which should remove the purchase details from the copies and allow them to be added to your iCloud Music Library. You need to use the same primary Apple ID in your iTunes Library and your other devices.

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If you add your own songs to your library when iCloud Music Library is enabled then they become available in just the way that your previous purchases and Apple Music tracks that you add to the library do. I have hundreds of original tracks by non-mainstream artists or even those by my son's band that aren't released anywhere, that are in my iCloud Library and have been uploaded to Apple's servers for my personal use. I can play them on my iPhone, iPad, HomePod, Apple TV and other computers that I own. Yes, if iCloud Music Library is enabled you cannot sync music items directly from your computer, but you shouldn't need to unless you have unresolved errors that you haven't addressed.

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Add the tracks to your iTunes library. See the link I posted earlier and make sure that the tracks you've added show up as uploaded rather than waiting, error, etc. Once that has happened if you look on your iPhone you should see the tracks in the library on your device ready to stream or download.

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