I can’t download music. I only have 5 devices attached to my Apple ID

Did you have other devices associated with your Apple ID in the past, that you may have to remove from the list?

Have a look at the steps in this related article, to find out:

View and remove your devices that are associated with Apple ID purchases - Apple Support

If you still can't download music and you did not use another Apple ID on your device in the past, you should use this link to contact the official Apple Support about that.

Only they can look into your personal details to find the issue:

Contact Apple Support

Or give them a call by selecting the phone number for your region from this Apple page:

Phone numbers Apple Support

So, in your case, in Download and Purchases/Managed Devices/ you see the message: "5 devices are associated with this Apple ID and can download purchases or subscription content"?

And if you click on "Manage Devices" all of your actual 5 devices are listed?