When I tried to book online for this Delta promotion, I got the following message:

We apologize...

but budget.com has not properly responded to your request.

You may use your browser's back button and resubmit your request now. In many cases, this second attempt will work. If the problem persists, please contact budget.com technical assistance in the U.S. at 800-404-8033 and we'll be happy to assist you.

The error code is 21-58-22022008-34068

I have written to ask budget.com customer service what the issue is.