If you get this error and have never joined this server before, you are likely using a free account. We do not support offline/cracked/free accounts. To join this server, you will need to buy an official Minecraft account from https://www.minecraft.net/en-us/.
If you have joined this server in the past, you can fix this error through the following steps:
Close all instances of Minecraft.
Open the Minecraft launcher.
Click on your account on the top left, then click log out.
Finally, log back in and reconnect.