Hey Glenn! Can you try the following, please?
In the terminal,
sudo nano /etc/mopidy/mopidy.conf. Scroll down to the bottom, and you should see a section that begins
[spotify]. Double-check that the username and password are both correct. If not, then change them to what they should be. Note that in my configuration I've used my actual username and not my email address, although I'm not sure if that makes a difference.
Also double-check on the settings page that you're logged in to Spotify correctly, and that it says "LOG OUT" next to "Authentication", rather than "LOG IN" and "limited access".
Hopefully that'll get you fixed up. :-)