Operation &1 of order &2 could not be deallocated

What causes this issue?

Operation &V1& of order &V2& could not be deallocated because one of the
activities is outside the planning horizon or within a fixing interval.

System Response

The system can only deallocate operations if all activities of the
operation can be deallocated.

How to fix this error?

Extend the planning horizon so that the activity is within the planning
horizon or change the fixing interval accordingly.

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.