If you are attempting to process Season 6 files, here are the standard commands used by the community: Convert Videos for FREE - FFMPEG
Rick and Morty often has "forced" subtitles (e.g., alien languages or background signs). If you convert to MP4 and lose subtitles, you can extract them first: rick and morty s06 ffmpeg
file 'Rick.and.Morty.S06E01.mkv' file 'Rick.and.Morty.S06E02.mkv' file 'Rick.and.Morty.S06E03.mkv' If you are attempting to process Season 6