Unable to download music

Well, then I would recommend reading over the whole iPhone user's guide.

Yeah, if you click that pop-up, then you will be subscribing to that Apple Music thing I talked about earlier. The first three months are free, after which they charge you monthly for the service. For help with Apple Music, see this link: Apple Music - Official Apple Support

I didn't understand the question you presented there at the end...

You can view your current subscriptions here to see if you actually subscribed: View, change, or cancel your subscriptions - Apple Support

If you've forgotten a component of your Apple ID, click the appropriate link below:

If you forgot your Apple ID - Apple Support

If you forgot your Apple ID password - Apple Support

Sure! 🙂 All of the music that you purchase is available for download in the iTunes app. This article should answer your questions about how to add music: <article redacted since it was the wrong one> 😉

Are you a new iPhone user?

This link IS the right one: http://help.apple.com/iphone/11/#/ipha48b73f47

You can download all of your old purchases in iTunes under More>Purchased. You cannot download any music in the actual Music app unless you are subscribed to Apple Music.