Enter an existing source assignment for source structure &3

What causes this issue?

You stored settlement profile &V2& in strategy sequence &V1&. Source
structure &V3& is entered in settlement structure &V2&.
Source assignment &V4& is not defined in source structure &V3&.

How to fix this error?

Use a source assignment that is defined in source structure &V3&, or
Extend source structure &V3& to include the required source assignment,
or
Enter another source structure in settlement profile &V2&, which has
the required assignment, or
Enter another settlement profile in the strategy sequence.

Error message extract from SAP system. Copyright SAP SE.