Changes made in the planning time fence will be ignored (& - &)

What causes this issue?

In Customizing, you determined that no new schedule lines can be
created in the planning time fence, nor can existing schedule lines be
changed. If a change occurs within the planning time fence, the system
displays an error message.

System Response

New schedule lines are not created within the planning time fence.

How to fix this error?

Check the dates of your schedule lines or check the planning time fence
in Customizing (Demand management -> Indep.requirements -> Fixing).

Error message extract from SAP system. Copyright SAP SE.