Only one "apportionment" task can be entered in task group &1

What causes this issue?

<DS:GLOS.task_group>Task group</> &v1& has more than one
'apportionment' <DS:GLOS.task>task</>. However, one apportionment task
suffices.

System Response

Processing terminated

How to fix this error?

Define only one apportionment task within one task group.

Error message extract from SAP system. Copyright SAP SE.