Partner role & is a mandatory role

What causes this issue?

The system has checked the arrangement partners for rebate arrangement
&V3& (e.g. for the functions "Extend" or "Create with reference").
In the process, it discovered that partner role &V1& in partner schema
&V2& is a mandatory role.

System Response

The function you invoked was performed successfully, but rebate
arrangement &V3& is incompletely maintained.
This may also apply to the rebate arrangement created by extension or
referencing.

How to fix this error?

Please maintain partner role &V1& for the rebate arrangement.

Error message extract from SAP system. Copyright SAP SE.