That server is either down, has the wrong version, or your server has closed. Try using the server's version, or ask the owner if the server is down. If none of these work, your server is whitelist-only. If you are a server owner, and want to fix this, continue reading. You should first go to the minehut console for that specific server. Once there, at the bottom of the server's console page (home console,) scroll down to where it says "whitelist," and check to make sure whitelist is off. If you want whitelist on, simply run the command 'whitelist {username}' on your console. You can enforce whitelist on the toggle next to this.