Multi-level product group not suitable for transferal

What causes this issue?

You have tried to transfer SOP data from a multi-level product group to
Demand Management. A multi-level product group is one whose members are
further product groups.

System Response

This is not possible.

How to fix this error?

Transfer data from single-level product groups. A single-level product
group is one whose members are materials.

Error message extract from SAP system. Copyright SAP SE.