What causes this issue?

The customer that you entered is a one-time customer (one-time
account). One-time customers do not have a permanent address.
Therefore, they are not allowed as reference in the transportatation
connection point.

System Response

You cannot make any further entries with this customer.

How to fix this error?

Choose a different reference or enter an address for the transportation
connection point.

Error message extract from SAP system. Copyright SAP SE.