JongmesHotfeld

1) Whitelisted server. There's no username corresponding to the one you entered.
2) The server itself is made private with a password (server password field)
3) Anything from the server you're attempting to join doesn't exist to you're on the wrong version of the game to you're trying to join a Steam game with a non-Steam game (GoG, pirated .etc).

When you click on a server, Theres A Sequence of Boxes labeled "Username" and "Password" on the right side of the server browser. You type a username, any username, into the box and then Proceed to make a password, Then Click "save". Then play.