ASN is not editable after posting the GR by buyer.
For example, PO is created for 10qty, ASN is done for 8Qty. but the qty received is 4, so accordingly GR is posted for 4qty. Now supplier is trying to edit the ASN and it is not allowing to edit. Kindly find the attachment for your reference
The reason you are not able to edit the ASN is because a follow-on document was created against it (in this case a goods receipt) .One option would be to reverse the Goods Receipt posted for that ASN and then check if you can edit the ASN and Submit. The buyer would obviously have to post a new GR.