Please always give us the exact version number of Audacity (all three numbers) not just say you have the latest version.
As you can see from the sticky topic at the top of this board Audacity 2.1.0: FFmpeg required for MP4/M4A/MOV/ALAC import unfortunately 2.1.0 requires FFmpeg to import MP4 videos. That is already fixed in our source code so it will be fixed in the next 2.1.1 release of Audacity.
If you see that problem again please could you give us steps to reproduce the problem.
Note that if you run out of disk space then exported files will be truncated. If you export WAV or AIFF files Audacity will warn you if the exported audio was not the same as the input audio.
Gale