If you are finding that you can not interact with the server despite registering an account, it may be due to your in-game username being entered incorrectly on the website (or not updated after a Minecraft username change)
To remedy this, you'll need to update your username on the website.
First of all, log in to your website account and navigate to User Settings.
Scroll down to the section Minecraft Username, make sure the username is entered exactly the same as it is in-game.
Once that is completed, click Save Settings. If you are logged into the server you will need to log out of the game server and back in again.