Welcome to the forum.
Transcoding errors can be caused by several factors, such as gaps in the Video (that are not "covered up" by Titles, Videos, or Stills on a higher Video Track), by problem Clips, or corrupt Clips, or other, Project-specific causes.
Since PrE uses a 2-pass Transcoding process, where 50% is the Duration of the Timeline, through the first pass, with a % of 15%, I would look at about the 7% mark on your Timeline. What do you have there? It could possibly be, say an MP3 Audio file (can cause problems), a gap in the Video, an overly-large Still Image, a Clip with different properties, than previous Clips, etc.. I would also search the Timeline carefully from that point on, as if there are, say gaps, then you will likely get another failure.
A screen-cap of the Timeline around that point might be useful, as well.
Good luck, and please let us know more about the Assets on the Timeline,