Dialed in through Skymiles Customer Service (888-750-3284) and selected "on Delta.com right now" option.

Within about 10 minutes, an agent came on who was able to re-create the itinerary on his end and put the transaction through.

He had no clue as to why it wouldn't work online for me (other than he thought the system was thinking "redeem by 9/14" might be "travel by 9/14".

Thanks for the replies.