just one thing to add.. the username is CASE SENSITIVE. I recently got the iphone 7 and when trying to login my phone would automatically capitalize the first letter of username - bad! I was getting the same error you are having (username not recognized, etc.). pay attention to the capitalization of your username.
Can't get Spotify premium to take money off my Apple Store credit. I have over 30 dollars on there and it won't let me listen to music without commercials. Plus it won't allow me to login after I logged out now I'm trying to login again and it won't allow me to do it. Plz help thanks