This error is happening because a piece of metadata is in use that hasn't been added to the main metadatabase (File>Meta). This usually happens when custom models are created for skins because "OFFSET" isn't a default metadatabase entry. Just go to to File>Meta and right click in the window that appears, then choose "Add", then add the piece of metadata you used that isn't in this list. Example: File>Meta, Right Click>Add, "OFFSET", Add. Upon adding all metadata thats in use, you'll be able to successfully save your pck.
Sometimes 4j Studios updates data in their pck files, so some updates will overwrite pck files. Don't worry, all you have to do is reinstall the pck when this happens.
Although all pck files are in the same format, they all have different structures. Some structures are a little more advanced because of some extra data 4j Studios added. As of now, it still hasn't been figured out. An example of one of these pcks is the Story Mode pck. Don't worry, you can just create a new pck (File>New) if you need a template. This could also mean you're trying to open a corrupted pck.