Entry of supplier not allowed

What causes this issue?

Supplier &V1& was entered for this item. However, a special stock
indicator was not entered.

System Response

The system cannot post this item.

How to fix this error?

If you want to enter a count for the special stock, enter the
appropriate special stock indicator.
If the supplier was entered by mistake, you can delete it in the
"Supplier" field in the IDoc editor and then post the IDoc again.

Error message extract from SAP system. Copyright SAP SE.