If you are receiving this error message it's because the location the product has been allocated from does not exist anymore due to it being deleted.
To fix the issue you will need to transfer the affected inventory and product to a location that does exist, once this has been done you can Unallocate Stock from the Bulk Actions drop-down menu and wait for the system to automatically reprocess it.
Note: More about transferring inventory can be found on the Transfer Inventory article.