Hey @cerise41, thanks for posting!
So autoplay is unavaliable on desktop and web versions of Spotify, but is avaliable on the iOS and Android versions. Here is a post by Loxer explaining this as well as providing a link to the idea page where you can vote if you would like to see it back.
On mobile devices, the autoplay feature can be found at:
Your Library > Settings (gear in the top right) > Playback > Autoplay