Sharon has the answer.
Unfortunately, it's not the greatest.
Vouchers, being animals made out of paper, can only be tendered to animals who know what to do with an animal made out of paper.
So, here's what you need to do. Take whatever fare you got off the web site and print it out. Then call Delta's reservation desk and ask them what the fare for the same itinerary would be through them. If same or less, fine skip the rest of this paragraph. If more, tell them, "Well, I am going to use a voucher, and I got this rate off the web site, will you honor it?" Probably they will.
Then just tell them, I want to pay for that in part with a voucher. (They view vouchers as a form of payment.) They'll say that they can put the reservation on a 24 hour hold (actually till midnight tomorrow, so if you call at 6 AM today, it's almost 48 hours).
Then drive to the airport and ticket the reservation using the voucher and whatever your credit card fancies are for the balance.
Yes, they do make them hard to use. This is partially by design. They hope that you'll just forget about it and pay full price.
But be persistent, and they will honor it.
And, by the way, DO NOT try to rebook that ticket. If you do, the voucher value will be GONZO! (as in, gone.) (I found this out the hard way when I got sick with something the doctors didn't know what it was and had to reschedule a trip paid for in part with a voucher.

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Have a great time!
Regards,
Joe Hurley
going on 11/9!