Pair formation: End message for start time &1 &2 is missing

What causes this issue?

You have tried to create a finish-time event. The system ascertained
that for an existing start-time event on &V1& at &V2& there is a
missing finish- or interruption-time event.

How can this happen?

The following time events exist in the system:
Start processing 8:00
Partial finish processing 9:00
Start processing 13:00
If a "Finish processing" with 15:00 is to be created then this is not
permitted since a time event is missing between 9:00 and 13:00 which
represents the end of the time period that started at 8:00.

System Response

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

How to fix this error?

First create the missing time event.

Error message extract from SAP system. Copyright SAP SE.