Does anyone know what would cause this issue? We are trying to do a vendor return authorization from a drop ship purchase order

It's pretty nebulous, but there's some sort of issue with the project task and project combination selected. Could be that you're selecting an invalid combination, could be things being inactive, could be that tasks don't have resources assigned appropriately, etc., etc. I'd experience this error a lot at my last place and would have to take a deep dive into all the project/task combinations to figure out what was wrong until I figured out the most common 4 or 5 reasons for that occurring based on my setup, and would then immediately check for those first. I know it's not a great answer, but hopefully that helps a little bit!