Unknown encoder 'x264' in FFmpeg despite following proper installation instructions

Try this:

ffmpeg -stats -i uncompressed.video.mkv -vcodec libx264 -preset veryslow -pass 2 -an compressed.video.mk

or this:

ffmpeg -stats -i uncompressed.video.mkv -codec:v libx264 -preset veryslow -pass 2 -an compressed.video.mk