Invalid assignment for exporting parameters in rule &1

What causes this issue?

Invalid assignment for exporting parameter in rule &V1&. The cause might
be:
No exporting parameter in signature of rule &V1&.
Multiple exporting parameters defined in rule &V1&, but no exporting
parameter selected when setting rule assignment.
The exporting parameter selected when setting rule assignment doesn't
match exporting parameters defined in rule &V1&.

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 the signature of rule &V1&.
Check the selected exporting parameter in rule assignment.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.