Data point to add to the issues:
Primary Apple ID is
XXXXX@me.com, it pays for an iCloud 2TB plan, and family shared this plan with 4 members. It also pays for monthly AppleCare+ for my iPhone. No other sharing.
Secondary Apple ID is
XXXXX@gmail.com. I use it for apps and subscriptions including Apple Music student plan.
The Migrate Purchases option did not show up. I contacted Apple Support via chat and Ramona said it is because I have two active subscriptions under my
XXXXX@gmail.com, DJay Pro and Apple Music student. Apple support helped me submit a refund request to fully cancel both subscriptions immediately, rather than me cancel, and letting the subscriptions lapse after the payment cycle. That way I don’t have to wait for weeks.
The Migrate Purchases option showed up next day. Clicked it, then it provided an error “Unable to Migrate Purchases. One or both accounts is not eligible for purchase migration.” I reviewed the entire Apple documentation and cannot figure out what’s making me ineligible, so off to Apple chat again. Apple chat agent Luis said they cannot access my accounts over chat, so they scheduled a call back with a phone agent, as they can access my account.
Apple agent on the phone Sydney went into my account, and immediately said that since my media and purchases account
XXXXX@gmail.com has used Apple Music, it is ineligible for migration. This completely goes against the Apple website documentation, but she said this is true. I asked can Apple delete all my Apple Music data as I do not want any of them, she said this is not possible. She said that I should fill in the Apple feedback form
https://www.apple.com/feedback/ to let Apple know that this needs to change.
Hope this helps anyone out with the “Unable to Migrate Purchases” error. Maybe it is because you have used Apple Music on your media & purchases Apple account.