Reference to &1 of this type is not allowed

What causes this issue?

You have tried to enter an invalid reference to &v1& for this &v1&.
You can only enter a reference from a &v1& with SNP routing to a
&v1& with routing.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.