More than likely it's a client issue, I'm loading in again to see if I get it but most lua errors (in my experience) are caused by conflicting mods. Loading in causes no issues for me so more than likely client side. Try restarting gmod, if that doesn't work try verifying the integrity of the game files. If THAT doesn't work try deleting the lua file lua/entities/cod-c4/init.lua and loading back in as this will re-download it and possibly fix the issue