Saving to mp4 failing

Using Version 4.6.2 on iMac with plenty space on HD and 16GB RAM impossible to save my 5 minutes slideshow on format mp4 to iPhone, iPad and Apple TV…?
Rebooted the iMac but nothing will work?
Thanks for any advice.

So you have made a slide show, and when you try to Share (export) it - what happens? That is, how, exactly, does it “fail”?