Hi all, I've been havin issues as well in the past while trying to connect Spotify to Alexa and controlling it via app/desktop.
In the Android app, I was getting stuck at the "connecting" phase: in order to get it to work I had to unlink and re-link my Spotify account in the Alexa app. On the other hand, it was working flawlessly from the desktop version of Spotify.
At some point it stopped working on desktop as well and I tried every suggestion found in this forum/reddit/etc. (unlink & re-link, uninstall & install again, reset modem, ...) except one last thing that I tried and did work:
Open the Spotify app (referring to Android)
Open the listed devices tab (not sure about the English name, it's the icon in the bottom left on the reproduction page.
Tap the 3 dots on the right of the Echo device you're trying to connect to and select "forget device".
Tell Alexa "Spotify connect": Echo should appear again in your device list and connect succesfully
Thanks IacoMaic. I can now connect to Alexa after starting another account. The problem is now playing saved albums through Alexa. Are you using Android and can you play a saved album (playlists work) when connected to Alexa ?