Please enter an order number

What causes this issue?

You want to allocate the PM notification to an existing order.

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?

Enter the number of the order, within which the PM notification is to be
processed. If you do not know the order number, use the system selection
function to help you find it.

Error message extract from SAP system. Copyright SAP SE.