Are you running BTS 3.19? If not, the mod probably wants that, so download and run the 3.19 patch. If you have modded something in the Assets folder, you will need to put it back the way it was. If you don't remember what it was, you can try looking at the modification dates to find the one that you changed but you will still need to know what you changed. If you can't do that but are sure that the problem is the Assets folder, you could install BTS on another computer and then copy the Assets folder back to your original installation to replace the changed one. Failing that, I can only think that you will need to reinstall BTS. Perhaps someone else will have a better idea.
BTW, this is why if you want to change something in a file, you should (in the future) copy it to CustomAssets and make the changes in the copy.
