&1: Assign add-on fields to the projection (see long text)

What causes this issue?

You are using the following fields in the add-on structure: &v2&.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Since these fields exist in the header table and already contain values,
they belong in the projection. Fields from the projection are
transferred directly from the database and are available to the business
check modules for evaluation in the analysis phase.
Add-on fields are purely virtual fields. They do not exist in the
database. These add-on fields are determined at runtime (-> transient
fields).
Caution: For technical reasons, the projection structure is included in
the add-on structure. The add-on fields are in addition to the
projection.

Error message extract from SAP system. Copyright SAP SE.