Internal trading partner & does not exist

What causes this issue?

The error analysis of the defective record found that the internal
trading partner does not exist.

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?

If you no longer need the assignment, please delete the line.
Otherwise, create an internal trading partner and start the assignment
again.

Error message extract from SAP system. Copyright SAP SE.