You are banned for a Compromised Account, this means hackers got access to your Minecraft account to use it for example hacking on servers, Cubecraft blocked them from getting on Cubecraft so they couldn’t ‘hurt’ your account.
Follow the steps on:
https://appeals.cubecraft.net/
Enter your MC name, follow the steps in the appeal, (you can get your appeal code on the ban screen when you try to log in on Cubecraft).
After that you should be good to go play again on Cube :)