Welcome to the forum.
A couple of questions first, please:
What version of PrE are you using?
What OS version are you on?
Are you sure that you have inserted a blank DVD into your multi-drive?
If so, did you do that, before you launched PrE?
What brand of blank media are you using?
Good luck, and let us know a bit more, please.
Just going to add on
a. Is this a :"it worked before but not now" issue or "it never worked before" issue?
b. If you did a burn to folder (4.7 GB) for your DVD-VIDEO, does that go to a successful completion?
Looking forward to your follow up.