The keybind on pc is the "Z" key. IDK about JoiPlay. You'll want to change the keybind to something you can use. The official name for the button is the "action" key. So set the "action" key to some button you have.
Try using JoiPlay version 1.20.015 and the RPG Maker Plugin for JoiPlay version 1.20.19. These are the versions players have confirmed work with the game.
Check if the plugins folder is intact and make sure it is not missing anything. Alco check your version of JoiPlay. I think the issue might have to do with how JoiPlay supports UTF-8 encoding.