FFmpeg No such filter error
The 2.8 branch is too old. The loop filter was added on 2016-02-18. The 2.8 branch was cut from the master branch on 2015-09-05. Release branches do not get new features backported (only occasional bug fixes).

Don't use such an old FFmpeg. Compile or download a recent build.

You can see a list of available filters for your build with ffmpeg -filters.