I did call Delta. And their solution was to book it for me. However, calling Delta every time I want to book a flight is not convenient. They connected me to "online support" and I was told they had no idea what the problem was. Seems odd it would just start like that. I've attempted a dozen transactions and the same error comes back each time.

Ironically, I usually book my flights on Delta while ON Delta flights (GoGo Wi-Fi), so calling isn't an option.