Date in the planning time fence (& - &) deleted

What causes this issue?

In Customizing for Demand Management, you specified that the system is
to issue a warning if changes are made within the planning time fence.
The planning time fence represents a period during which the
<GL:demand_program>demand program</> is protected from any changes.

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

System Response

Schedule lines were deleted in the planning time fence.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.