Could you provide more details of the steps that you are taking? It appears that you have an old iMovie 6 project that you would like to burn to a DVD with iDVD on your Mac that is running Mojave, but you can't get the project to open? Is that correct?
iDVD has somewhat limited function on Mojave. Some of the themes don't work, and sometimes the burn won't get completed. The iMovie 6 app will still work on Mojave, as it does on my Mojave set up. iMovie 6 will not work on Catalina or later operating systems, because those systems are 64 bit and iMovie is a 32 bit program.
Take a look at this link and see if there is anything in there that helps you.
How far back does "Update Projects and Ev… - Apple Community
-- Rich