Booking a new ticket for myself. Got a similar message trying to apply an eCredit from an (agent issued) unused ticket.
Oh no! We're sorry, but we are unable to process this request online. For assistance please click the “Message Us” button on the Need Help page and a reservations representative will support you. #9002R
For context, the amount due from new ticket is less than the value of the eCredit.
Any experience/data point?