Further scheduling not possible until last call is confirmed

What causes this issue?

You are trying to schedule a
<GL:preventive_maintenance_plan>maintenance plan</>. This will not be
possible until the last <GL:maintenance_call>maintenance call</> has
been confirmed.

System Response

INCLUDE 'KEINE-FUNKTION' OBJECT DOKU ID TX LANGUAGE E

How to fix this error?

Complete all the <GL:maintenance_order>maintenance orders</> for the
last maintenance call.

Error message extract from SAP system. Copyright SAP SE.