Does this only occur in game? If yes
First of all make sure you`re logged in with the same Steam account which you were on when you registered at TruckersMP.
Open notepad or a text editor.
You need to write 2 rows, on 1st your email, on the 2nd your password.
Now, copy the email from notepad and paste it in the ingame login screen using keyboard shortcut keys (CTRL+C for copy, CTRL+V for paste - ~ Please notice that these shortcuts may be different in your country. If they don`t work, please inform yourself how you can do it on google ~).
Do the same for the password.
Login.
If the solution didn`t work, you must have a strange character in the password that can`t be recognized by the login system ingame.
To bypass this problem, you should change your account password to only latin letters (a to z) and digits (0-9). Please notice that by doing this your password might lack in strength.
In order to change your password, you must login at TruckersMP.com, press Edit Account, insert the new password twice, confirm the old password twice and then press save data.
I hope that one of these fixes worked and you`re now able to login ingame.
If they didn`t, please let us know, together with a brief description of what you did.