For the record, it appears this tware ended on time. 20 minutes after Twitter posting, I am getting:
We were unable to validate the promotion code or eCertificate number you entered. Please verify that the code or number is entered correctly and the promotion has not expired.
OP, for future reference, I believe you should post in Mileage Run Deals and the mods will move it to Mileage Run Discussion once the fare is gone