Yes. What Tero and Jon said. Now, I’m not sure why iDVD won’t take an h264 mp4, but you can make a couple other options. The image size will be changed as Tero suggested to 720×480. In the QuickTime presets you should have a ProRes NTSC SD preset that should work. If not, try a MPEG2-DVD NTSC DV Wide preset. They will crunch it down to the correct size. And iDVD SHOULD be able to use either of them. Unfortunately, my Mac, with Mojave, no longer has iDVD so I can’t say for sure.
Let us know what happens!
Good Luck!
TC