You must enter a partner number for the partner function

What causes this issue?

You have entered a <GL:partner_function>partner function</> for the
selection, but have not entered a corresponding partner number.

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?

Enter a <GL:partner>partner</> that has this function.

Error message extract from SAP system. Copyright SAP SE.