Click the menu button on the top left. Then click About. Once in the about section you should see three vertical dots next to the settings icon. Click on the dots, and you should see developer settings listed. See screenshot attached. Let me know if you're still having issues.
Tapping Version in the Settings menu seems to be the new way to do this or at least an alternate path to try if the other solutions offered don't work for you. Worked for me using a Xiaomi Redmi Note 7. Restarting both Android Auto and Spotify (if it was running when you made the change) was key for me to get it to work.
>>"Then in Spotify, there's an Autostart option where you can find "Allow the app to be started by other apps"<< - this chapter helpes me, not any dev mods and other, just simply start by other app works fine. Btw thank U ;)