Sales area (VKORG/VTWEG/SPART) is incomplete

What causes this issue?

You must always specify a customer number and a complete sales area for
a higher-level assignment when maintaining assignments in a
<GL:customer_hierarchy>customer hierarchy</>.
You may not specify a higher-level customer number or a higher-level
sales area if a node should not have a higher-level assignment.

Error message extract from SAP system. Copyright SAP SE.