user not able to attach proof of delivery document for an Drop ship PO

Summary:

Here user trying to attach proof of delivery document for an drop ship PO , but he is getting error while submit the changes.

Content (required):

Error: changes to this schedule must be initiate d from the associated sales order

Version (include the version you are using, if applicable):

21D

Code Snippet (add any code snippets that support your topic, if applicable):