Hello berlin3,
Thank you for reaching out in Apple Support Communities. It sounds like you may have exceeded the amount of associated devices under your Apple ID account, and you're unable to download music on an iPhone. We'll be happy to help and point you in the right direction.
You can view and remove associated devices under your Apple ID account by following the steps outlined here: View and remove your associated devices for purchases
After removing an associated device, please let us know if you're able to download music on your iPhone without any issues.
Kind regards.