premiere 2021 crash

I don't know how to figure out what caused the crash.  Video editors do that occasionally.  They all do!   There may be a rescue anyway.  Premiere Elements automatically makes backups.  I think the default is every 15 minutes and it will keep five.  In other words, you may have lost only 15 minutes of work!

Look for a sub-folder where your project is.  It will be titled "Adobe Premiere Elements Auto-Save".  In it will be numbered files.  

To be extra careful, make a copy of the latest one and double click the copy.  It should open in Premiere Elements and you can fix the last few minutes of your work and try exporting again.

movie renders perfectly, crash comes with trying to share using 720 format, error screen below.

I jsut upgraded from premier elements 11 - it never gave me any problems with movies like this one.

You need to provide details for anyone to help
Mac or Windows and exact version of the operating system
CPU brand and model and clock speed
Video adapter brand and model and driver version and amount of video ram
Number and size and type of hard drives
What kind of video are you editing?
-hint 4k video on an underpowered computer will not play well