convert audio to libopus with ffmpeg I'm totally new to ffmpeg and use it on windows.
I want to encode the audio from a mkv video to opus, using

ffmpeg -i abc.mkv -map 0:v -c copy -map 0:a -c libopus -b:a 128000 xyz.mkv


but I get the error:

invalid encoder type "libopus". 


What am I doing wrong?
Just -c on its own will set the encoder for all types of streams: video, audio..etc. libopus can't encode video, obviously. You need to attach a stream specifier to limit it to audio, so

ffmpeg -i abc.mkv -map 0:v -c copy -map 0:a -c:a libopus -b:a 128k xyz.mkv


See the section on stream specifiers and also this answer on how multiple codec settings in a command works.