Swosti,
As suggested by TW Sir its not possible to input "reason for rejection" after line item is delivered because subsequent document is created then line item becomes in display mode even in VA02.
But in some scenario it could happen e.g user has created a Sales document which is consisting 2 line items and later on created and delivery but here he picked only one line item and did "PGI" and later on if he doesn't want to deliver to 2nd line item created in sales document so in that case user may input "Reason for Rejection" for 2nd line item and sales document becomes complete.
Thanks
Kamlesh