A failure to complete DVD creation in FCPX can often be related to a corrupt render file in the project. Open the library by selecting it and "show package contents". Manually delete the render files for the project. Restart FCPX by holding Option and Command keys to trash preferences. Load your library and project and try to share the DVD again.
If you still have the original media on the physical card, or a copy of the card on your Mac, you can try using the Reimport command (File > Import > Reimport from Camera/Archive).
If you optimized the original clips, this error can sometimes be thrown because the transcoding process never completed. So if none of the above seems to fix it, try selecting the clips in the Browser , right clicking, and choosing Transcode Media.