Novated Purchase orders cannot be changed error

Hi Experts,

    while trying to edit the PO we are getting an error as " Novated Purchase orders cannot be changed" . I am confused that which field is used to differentiate the normal PO to that of novated PO at portal level.  Please let us know why this error occurs and how can we solve the issue ?

   Thanks in Advance.

Thanks & Regards,

Aishwarya.M