Hello @SylvanSpider355,
1-) Make sure to enter TruckersMP with the steam account you are using.
2-) Enter your email address and password in a text document, then copy and paste to the game screen.
3-) Make sure family sharing is turned off.
4-) Make sure your TruckersMP account is installed.
If they don't work;
You can create a support ticket: https://truckersmp.com/support/ticket/create
They will help you in a professional way.
Regards...
AhirZaman