We had this issue with my wife's Spotify account. We finally got it working. I don't think this will be the same as everyone's issues, but I thought it would be great to share.
"Sign up to Spotify with Facebook" caused our issue/confusion.
Long story short, my wife ended up having two Spotify accounts linked to the same email address. The family premium account she had signed up with the Facebook option, this used her email as a contact, but her username was a bunch of numbers (we assumed the username would be her email address).
At some point, she tried to sign in to Spotify on a new device via her email address, which mistakenly set up a new free account using her email address as the username. So both accounts use the same email address in the "email address field," but the username is different.
In the Tesla, we tried to sign in to the FREE Spotify account, which did not work. Once we worked out the issue with signing up to Spotify using the facebook credentials and finding out the actual username, we could log in to the Premium account.
Hope this helps.