Hello @MrWoodsman,
1) First check if the installed ProMods version is the correct version
supported by the ETS2 Multiplayer Mod (TruckersMP), in this case the supported version is 2.55.
Illustrative Images:
2) Next make sure you have all the Map DLCs as this is mandatory for you to be able to play
on Promods servers:
DLC Iberia (optional);
DLC Road to the Black Sea;
DLC Beyond the Baltic Sea;
DLC Italia;
DLC Vive la France !;
DLC Scandinavia;
DLC Going East! ;
3) If the version is correct and the error persists, check if extracting the files failed, if necessary
extract the files again to be sure.
4) Make sure the necessary files are in the correct game folder. The path may be different
on some operating systems but make sure the files are inside the ''mod'' folder.
c:\Users\<username>\Documents\Euro Truck Simulator 2\mod
5) Once completed, do not activate the ProMods Mods and do not open in Offline mode, just open the
Multiplayer TruckersMP Launcher and check if this error has been corrected. Hope this can help you.