I was having this issue and this was the cause:
All of my inventory was set to the address I had originally put in (mailing address) and it had defaulted to. I added a second address (fulfillment warehouse) and changed the default to that address but hadn't changed my inventory to that address.
To fix I deactivated my original address, and when doing that it prompted me to set a destination for all of my inventory to go to, at which point I selected the fulfillment warehouse.
Hope this helps someone!!
Im getting the same error and think it maybe to do with the 2 locations, so our unit or the fulfillment warehouse BUT not all items are at the warehouse so we set some to unit also, i think thats the issue but cant set all to warehouse as it will show zero stock with them so cant sell....its a nightmare must be some easy fix?