Errors in .mp4 file after exporting video

Bill, thanks for taking the time and energy to respond to my question and thanks for patience in the time it has taken me to respond.  Based on your response and the questions you asked, I went back and began troubleshooting.  First I tried excporting the video on a different computer, just in case it was something with my hardfware.  It wasn't.  So then I went back to your questions.  My video is audio with a collection of photos, so I deleted and replaced the two photos where the problems were, then redid all of my transitions, discovering that there were some oversights there.  Once I did all of that, I rendered again (had to learn how to do that, lol, as I am a complete novice at this), and my export to a .mp4 was perfect. So thanks again for your time and questions.  I learned a lot in the troubleshooting process and I really am grateful for that as this is the first of many videos to come.  Have a great day!