Customer & is not linked to vendor &

What causes this issue?

Vendor &v2& references customer &v1& as the exchange partner
but the customer &v1& does not reference &v2& as the exchange
partner.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Check that you have entered the correct exchange partner.
If you have entered the correct exchange partner then make the
appropriate changes to the customer record (control data screen)
so that it references vendor &v2&.

Error message extract from SAP system. Copyright SAP SE.