The THINGS lump is the first lump (file) in any map. If the game can't find it, that means it probably can't find the rest of the map's data either. How are you organizing your addon? Are you using a PK3 for this pack, or a WAD? If you're using a PK3, there is a specific folder structure you must follow, otherwise this error will happen. Bear in mind that I need to know what you're using for the whole pack, not just the map.