The problem is probably the version of the plugin you have downloaded.
https://www.patreon.com/posts/joiplay-1-20-611-137694579
This is the official Joiplay Patreon account. Here you'll find the most up-to-date versions of the plugin that aren't available on the official website. Just look for the option Ren'Py 8.4.1 Plugin 1.00.10 and then click on Direct. I don't know if you've already solved the problem, but if not, I hope this helps ^^