Smart Form and PDF-based form not allowed in same output request

What causes this issue?

You want to generate multiple documents using the current output
activity. The documents are based on different forms, and one form was
set up as a Smart Form while the other was set up as a PDF-based form.
However, the use of both Smart Forms and PDF-based forms within the same
output activity is not supported.
The form &V2& of correspondence activity &V1& does not have the same
form type as the previous form in the output activity.
You are only allowed to use either Smart Forms or PDF-based forms within
one output activity.

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.