I know this is going back to 2016 but just for anyone that has the same fault I had the same error and it was the email address I had saved on Spotify wasn't the same as I was using for Alexa and when I updated so that both were the same it connected ok and all working ok now :)
This error continues to happen CONSTANTLY. For some reason my two echo dots seem to connect more regularly and are viewable in the available devices list (but not 100% of the time) while my brand new 2nd gen Echo is almost never available or drops out of availability in the middle of listening to a song.
The only solution I can find is to unlink and relink the Spotify account in the Alexa app, but I have to do this constantly. I’m talking every single time I want to play music only Echo.
Im highly disappointed in this software. It’s making me want to return my Echo and cancel my Spotify. Honestly, very disappointing.