Version & is not a delta version in controlling area &

What causes this issue?

You want to assign your own business transactions to a delta version in
Activity-Based Costing for version &V1&, controlling area &V2&. However,
the version is not a delta version in controlling area &V2&.

System Response

The system prevents navigation to business transaction assignment.

How to fix this error?

A version is a delta version if it uses another version as a reference.
You can create a delta version in the general version definition for a
given controlling area by entering a reference version.
Afterwards, you can assign the business transactions you require in the
given delta version.

Error message extract from SAP system. Copyright SAP SE.