You cannot cancel the document

What causes this issue?

The goods movement material- material cannot be cancelled since the
quantity to be posted was assigned to indiviual purchase orders when
the goods movement document was posted.

System Response

The document cannot be cancelled.

How to fix this error?

Instaead of a cancellation, execute another material - material
posting.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.