Vendor & Does not Exist for Company Code &

What causes this issue?

The vendor account does not exist on the customer master for the entered
operator.

System Response

Processing cannot continue until an operator with a vendor account is
entered.

How to fix this error?

Check the customer master (table KNA1) to ensure that the vendor
account field is filled in with a valid vendor account.

Error message extract from SAP system. Copyright SAP SE.