This is a hard one usually since it can be anything that has with the map to do.
But something to go with is usually that you've used items/objects or other custom files for the map, that now doesn't exist where they should've been, so when the map tries to export and look for the raw files to put into the finished .fhm or .fho and doesn't find it. It fails to do so. This is why it's important to not move around or delete any files in your game folder that a map might use that you are working on.
Maybe someone else knows exactly what to look at when you get this message, but I myself never got anything worse than a few failed exported objects.
