Why can't my friend join?

My friend and I wanted to play Minecraft together, which has worked well so far, but at the moment he is getting this error message.

He is from the United States.

My plugin list:

CoreProtect, FastAsyncWorldedit, UltraRegions, UltraPermissions, ServerBooster, ClearLag, Ultra Punishments, Viaversion, StaffEssentials, UltraPermissions, ProtocolLib, CommandSpy, Vault, MyCommand, DiscordSRV, Essentials, EssentialsChat, PlotSquared, EssentialsSpawn

Does he have the same version as you? What version is the server running on?

The server runs on 1.12.2

Well then he has to join with 1.12.2. I see you have ViaVersion on it. That didn't work for my network either.

I also joine with 1.14.4

Then just try to join both with 1.12.2 on it. It's not difficult.

The "You are not capable." Text is not standard text, but a special kick message from one of your plugins.

Look if he joint in the console, there should actually be which plugin prevents his join.

If not, try to find out the culprit by always partially deactivating the plugins until he can join.