PSA: If you are trying to migrate purchases (https://support.apple.com/en-us/117294) and are a developer, check that the "secondary" Apple account isn't a user in App Store Connect.

*That* was a fun adventure.

Migrate purchases from one Apple Account to another Apple Account - Apple Support

If an Apple Account is only used for making purchases, those purchases can be migrated to a primary Apple Account to consolidate them.

Apple Support