Can't convert FLAC to MP3 with FFmpeg
Your version of FFmpeg has no MP3 encoder. It can only decode, as indicated by the "D" in the command line output.
If you built it yourself, install LAME before — this depends on your distribution — and then reconfigure it --with-libmp3lame.
Or pick a static build from the FFmpeg download page.
This should give you access to the de facto standard MP3 encoder.