Mmmmmm......."Your itinerary may not be eligible for an upgrade.
Please ensure to use your 6 character booking reference”....
I did use the correct PNR. Perhaps affected by an earlier sector booked on a MH/JL code share?? Any thoughts?
Possibly ... Was the ticket issued by MH? If so, the ticket number should begin 232.
How did you purchase it? Others have reported that you can't bid for an upgrade if it was bought through a (O)TA, but that doesn't appear to be the case all the time.
When did you book it? It can take a few days before the booking is eligible for an upgrade ... or so it seems to me.
I would suggest ringing MH.