Final Cut Pro 10.7.1 gives me an error message when I try to export project into H.264?

I had a similar error recently when doing a quick 'n dirty edit on old .mp4 movies for my son (I did have the original VHS converted to .dv footage archived on HDD ... lazy me). Anyway, I fixed it with:

In FCP Preferences > General set Time Display to "Frames" or to the same format as in the error report.

In the timeline navigate to frame [n] and see what is there (zoom in enough to see individual frames). Delete that frame (with arrow keys and b blade tool). You might get away with just deleting all render frames, and turning off background render.

...I later noticed that there are many such dropouts on those .mp4 delivery movies. Maybe I should again re-encode them from the .dv files when I get a faster Mac... Or maybe those VHS dropouts plague me even then...

This usually indicates a problem with the media.

Try deleting all render files.

Turn off "background render" in Settings->Playback.

Any change?

Maybe the very first clip in your timeline has a problem.

Try exporting a range. Does that work?

Try exporting a new test project. Does that work?

Ah! So the problem is only when you export "Video and Audio" and choose H264; but choosing "Computer", which produces an mp4 file (incidentally using H264 also) works?

Do other exports, like "Apple Devices" work?

Try deleting FCP preferences. To delete preferences, hold down command and option while starting the application and click Delete Preferences.

The application will start in a default state, and open an "Untitled" library. Reopen the library you were working on, set the preferences the way you like them, and try again.

If the problem persists: Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

The MPEG-4 export is H.264.