Have you already tried logging into the Spotty plugin via Spotify Connect? Are you using the latest version of Spotty? Can you find any information about the error in the server log?
Got it working I had a free account and then had to sine up for a Spotify Premium account. Had to put my Order ID in and it works now. That’s how have to have Spotify working on Max2play. It says get 2 months for free.
Had to do a apt install flac and rebooted it and now it all works.