Assuming you're on android?
Guessing it's the standard issue; using the broken current version of JoiPlay, 1.01.430
You can either downgrade to 1.01.400 (https://apk.support/download-app/cyou.joiplay.joiplay/1622358000_vs-1.01.400) and disable auto-update in the playstore so it doesn't immediately replace it with 1.01.430
Or go into JoiPlay playstore settings and enable beta versions, and use 1.01.500
If problems persist, try the JoiPlay Discord: https://discord.gg/QEeYqNGt