The reason:
Your ffmpeg is too old.
The -var_stream_map option was added on 2017-11-20 in commit 92a32d0, but the FFmpeg 3.4 release was created on 2017-10-11. New features are not added to releases, so that is why 3.4.2 also does not include this option.
You can download a recent ffmpeg version from the git master branch, or compile.
