Ronal I believe this error has happened because you had your server running, joined your server, added the plugin, and then typed "/reload" or something of that sort. The plugin needs to be loaded on it's first go with NO PLAYERS in the server meaning a reload will likely break it. You need to do a full shut down and full fresh startup. To fix the issue I believe you may have, simply stop the server, delete the plugin's files, start the server again completely fresh, and then allow players to join. It should fix everything. If it doesn't fix it feel free to reach out on my discord server:
https://discord.gg/H32FcPp
I don't check spigot often.