H264 Export Fix frame .. export error

Ok but what are the tech specs of your computer? is your system up to date? what version are you using? a little bit of that information wouldn't hurt. You could try to Interpret Footage to switch to constant framerate within Premiere Pro if they are as variable framerate.  

by any chance were any clips from a screen recording or shot on a smart phone.  If so, the problem is probably that they have a variable frame rate.

use mediainfo to determine whether your source is variable or constant frame rate

https://mediaarea.net/en/MediaInfo/Download

if it's variable use handbrake to convert to constant frame rate

https://handbrake.fr
and here's a tutorial on how to use handbrake

https://www.youtube.com/watch?time_continue=34&v=xlvxgVREX-Y

try to export using 'Software Only' renderer