Hey bloo1dy,
The error message you're seeing usually happens when trying to use ForceOP on a regular ("premium") Minecraft server, where you need to have a real Minecraft account in order to join.
ForceOP (a.k.a. "AuthMeCracker") is only meant for cracked servers that have AuthMe or a similar plugin installed - the kind of plugin that prompts you to type /login
and /register
in chat. It won't work on other servers because they don't allow you to join with someone else's username.
Hope that clears things up! If you do happen to see this message on a cracked server, let me know and we can troubleshoot further.