What causes this issue?

You have changed or created a schedule line in such a way that an open
schedule line is now placed before a consumed schedule line.
The following example illustrates the situation:
The delivery schedule contains at least two schedule lines; A and B,
where B describes the changed or created schedule line.
Schedule line A has a goods receipt (GR) or an advanced shipping
notification (ASN) booked.
Schedule line B has an open quantity and an availability date before
line A.

System Response

In the active version liveCache automatically reassigns the quantity
consumed by goods receipt or ASN to the oldest open quantity.
In the above example liveCache would reassign the GR/ASN-consumed
quantity to schedule line B and schedule line A would be open again.
In the simulation version the schedule lines are stored as is and the
system does not reassign consumed quantities.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.