Hi, I'm HuangZhiRong,I have a question to add：

Manually add thumbnails is feasible.
when I go to the plugin page 'Administration' -> 'Plugins' -> 'VideoJS' -> 'Synchronization'->Run the Synchronization.Select the option as Only simulated operation(Will not change any content in the database) .No error occurred,and no thumbnail.I can see the PWG_representative folder built, there is no file inside.

The error message indicates what you need to do.

Error poster running ffmpeg/avconv, try it manually, check your webserver error log:
ffmpeg -ss 4 -i "./upload/2016/12/19/*.mp4" -vcodec mjpeg -vframes 1 -an -f rawvideo -y "./upload/2016/12/19/pwg_representative/*.jpg"

1) Check your webserver error logs
2) Try it manually -> ffmpeg -ss 4 -i "./upload/2016/12/19/*.mp4" -vcodec mjpeg -vframes 1 -an -f rawvideo -y "./upload/2016/12/19/pwg_representative/*.jpg"

The error indicates that ffmpeg is unable to execute.  By doing the two steps above, you should be able to determine why this is so.