Hello,
1) As you´ve already tried to install the mod, there will be a folder called TruckersMP in Program archives
2) Once you´ve found it, create another one called TruckersMP (put something here to know this is the good one, like 2), in Program Archives as well.
3) Try to uninstall ALL you can in the TruckersMP default folder, the one you first installed and is giving you the error.
4) Once you´ve uninstalled all in that default folder, Install again the mod in the TruckersMP NEW FOLDER. It should give you no error
Hope this will help you ^^