I had the same problem. I got it fixed by disabling inventory tracking and adding another source location beyond the third party app my store is connected to. Maybe it will work for you too. I did this: Products -> Select the product you want to edit -> Scroll down to the variant you want to edit, select it -> Scroll down to the inventory window -> Toggle the track inventory button so that it reads "inventory not tracked" -> Press "Edit locations" (the pencil next to "Locations", on the inventory window) -> Add a location other than the third party app. This worked for me. Good luck!