Hi, scrubrnmom,
To burn a DVD, share out to your desktop your movie from iMovie, then drag it into the upload window of a DVD authoring program, or upload it in accordance with the DVD app's instructions. You can burn it from there. Use a standard single layer DVD-R disc and burn at 4x speed. You will need a DVD burner, either internal to your Mac or external to do the burning.
Unfortunately, iDVD is very dysfunctional on Mojave. You might want to consider purchasing a DVD authoring program made by Peng GUIPING and is called DVD Creator and is available in the app store for $14.99.
Here's a link to a discussion about replacement software for iDVD. Pay particular attention to the post by Batman15:
https://discussions.apple.com/thread/250714476
-- Rich