If you're encountering issues connecting to Spotify in order to access your premium content, follow the steps below:
Often times, the cause of this disconnect is a discrepancy in the username if you are using Spotify on more than one device. To help with troubleshooting could you follow the steps below:
FOR IPHONE
On iOS, if you go to Settings --> Account you will see something like this (this is iOS): (pic 1 below)
Then, on the web, you can visit https://www.spotify.com/us/account/profile/ (or your local equivalent, which you can get to by visiting https://accounts.spotify.com and clicking "Account Overview").
If you click into "Edit Profile", you can see the username: (pic 2 below)
As you can see in the two screenshots, the two Usernames don't match. If they don't match for you, sign out on your secondary device, by finding your avatar in the upper right.
Then re-log-in with the username that you find on the device you use primarily for listening: