You might wanna tryout updating your DLC's as well, might be an issue with that as well. You can do that here: https://truckersmp.com/profile/settings
Other than that I think, as Tim2060 already wrote, you might be signed into a Beta or some other version which is not supported. To fix that, go to your Steam Library, right click ATS, click on "Properties" and then on "Betas", make sure you're signed out of all.