I think I know what the problem is.
Open Windows Explorer.
Go inside C:\Program Files\Buena Vista Interactive\Tron 2.0\Custom\Mods. This is where you need to look if you installed the game to the default location.
Look at the folder inside there. Make sure it's UserError with NO SPACES.
Go inside the UserError folder. Check the file in there, make sure it's named UserError.REZ. Again, NO SPACES.
The folder and .REZ file must have exactly the same name, and no spaces. That should cure your problem.
Next time, you can use REZ Handler to install a mod to avoid this type of problem again.