Run as an administrator. Verify game files. Click here for more information: https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335
Try logging in again by deleting My Documents> ETS2 or ATS> Config.cfg.
Start the game as Directx 64 BIT. Click here for more information: https://forum.truckersmp.com/index.php?/profile/495-rested/&status=38604&type=status