Why do I receive "Skip because unable to find value in source field" error?

Introduction

The error message appears when the Source Field of the triggering issue does not have a value.

Given the following post-function configuration:

If the Assignee field of the triggering issue is Unassigned,

then you will receive an error message

To fix the problem, please set a value in the Source Field of the triggering issue.

E.g. Set the Assignee field to Leon