Partner role & is a mandatory role

What causes this issue?

The system has checked the contact persons for rebate arrangement &V3&
(the arrangement number is missing in the case of the function "Create
rebate arrangement").
In the process, it discovered that partner role &V1& in partner schema
&V2& of arrangement type &V4& is a mandatory role.

System Response

It is not possible to save rebate arrangement &V3& because it is
incompletely maintained.

How to fix this error?

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

Error message extract from SAP system. Copyright SAP SE.