Hai,
Follow instructions below
1) Enter to the site with E-Mail, which you register on the site, not your game nickname, or Steam nickname;
2) Write your E-Mail and password in Notepad, and then copy them one by one paste in the login window, if you not sure that your login and password is correct writing by your self;
3) If none of the above advice does not help the situation, try to reset your password and log in again;
4) Do not use a password or E-Mail any symbols, password must being only characters and numbers;
5) Do not use Steam-account, if the game is not bought on it, and on the other account. With the «Steam Family Sharing»to play in multiplayer you cant to connect also ;
6) Your e-mail address from a Steam account must match the email address used when you registered on TruckersMP site. This is a very important point!
7) Run Steam, then press Profile> Edit Profile> My private setting and set a to all point is "Public";
8) And if you do not help any one piece of advice, ask for help to the support team with a detailed description of the problem.
Cordially,
Mirko