This forces a constant frame rate (CFR) of 30fps. This is the first major step toward playback.

You can use a hex editor or a simple command-line tool to check for the position of the moov atom.

This usually means the input file is corrupted or was not a complete MP4. Try running ffmpeg -err_detect ignore_err -i input.mp4 -c copy output.mp4 to attempt a fix, though it may result in glitches.