&1 change in item &2 will unassign PO item

What causes this issue?

The material is relevant to Supply Assignment (ARun) and an assignment
exists for this document item.
Item will be unassigned for one of the following reason.
Unassign rule is not defined to specify the date horizon against
Purchase order in customizing under <LS>Cross-Application Components >
Advanced Available-to-Promise (aATP) > Supply Assignment (ARun) >
Cross-Applications > Purchase Order Handling</>.
Unassign rule is defined in customizing under <LS>Cross-Application
Components > Advanced Available-to-Promise (aATP) > Supply Assignment
(ARun) > Cross-Applications > Purchase Order Handling</>. You have
created an inbound delivery for an assigned purchase order item with the
inbound delivery date greater than the item delivery date. Therefore,
the assigned requirements with respect to purchase order item is
compared against the inbound delivery date. If date reaction is outside
the specified horizon, the purchase order item is unassigned.(in this
case, only the purchase order date horizon is considered and not the
date reaction field check).

System Response

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

How to fix this error?

Define a rule to unassign orders. You can specify how the system should
react when the document is modified (for example, change in range of
days for purchase order).
Enter an alphanumeric key (maximum of four characters) and a description
for your rule.
Check the date horizon parameters maintained and change if needed.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.