Ah-hah! At least you got my eyes to read the fine print......
My milage upgrade request was borking the option for using a cert.
Note: Due to the milage redemption required for an upgrade, only a credit/debit may be used to purchase this reservation with an uprade.
I don't know why that is true....seems like a nonsensical restriction to me.... Why would using miles as an upgrade (in addition to paying the $$ fare) make using a ETC invalid? ........ but there it is.
I guess I'll just put through the ticket then call for the milage u/g