error:requestcvpixelbufferforframe returnred 3 for absolute frame 26679  

The error message is saying that there is corruption at frame No. 26679 that caused the export to fail. It is probably a black frame. Expand your timeline full out and slowly scroll through it in the area of frame No.26679. Look for black frames, white flashes, artifacts and any other indications of corruption. Cut out any that you find and export again.

You can find where frame No. 26679 is located by dividing the frame rate of your project into 26679. That will tell you the time point in seconds at which the frame 26679 appears. So, for example, if you have a 30 fps project, divide 30 into 26679 to get 889.3 seconds. Divide 889.3 seconds by 60 seconds/minute to get 14.8 minutes as the place in the timeline where frame no. 26679 appears.

If you want to preserve your original project, you can duplicate the project and work with the duplicate.

-- Rich