You cannot delete all schedule lines for item &

What causes this issue?

The system could not delete all the schedule lines for item &V1&. If
the schedule line is linked to a procurement element (purchase
requisistion or assembly order) this could be because the procument
element cannot be deleted.
A purchase requisition can have a release status that prevents it from
being deleted. For example, an assembly order cannot be deleted if it
is partially or completely confirmed.
This message also appears if the system stops deleting in the SD
document.

How to fix this error?

You can try to change the procurement element in the detail screen for
schedule line procurement so that the schedule line can be deleted. If
this doesn't work, inform the purchasing or production planning
department.

Error message extract from SAP system. Copyright SAP SE.