Specify an outline agreement number in source list

What causes this issue?

The system has found a source list entry for material &v1& in plant
&v2& for vendor &v3& (optional) that has no maintained outline
agreement number (purchase scheduling agreement number).

System Response

Processing is not possible.

How to fix this error?

Maintain a purchase scheduling agreement in the appropriate source list
entry.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.