FFmpeg seems to be unable to transmux when remapping streams

Question

I'm trying to reorder a video's streams in the form of video streams first, audio streams and then subtitles. I thought I could also change the format of the video from mkv to more common one like mp4 but no matter what I try it still shows this error message:

Input #0, matroska,webm, from '/path/to/file.mkv':
  Metadata:
    encoder         : libebml v1.3.6 + libmatroska v1.4.9
    creation_time   : 2019-06-17T20:41:37.000000Z
  Duration: 01:03:10.17, start: 0.000000, bitrate: 1215 kb/s
    Chapter #0:0: start 0.054000, end 3790.174000
    Metadata:
      title           : Capitolo 01
    Stream #0:0(ita): Subtitle: subrip (default)
    Metadata:
      BPS-eng         : 4
      DURATION-eng    : 00:52:26.144000000
      NUMBER_OF_FRAMES-eng: 37
      NUMBER_OF_BYTES-eng: 1627
      _STATISTICS_WRITING_APP-eng: mkvmerge v30.1.0 ('Forever And More') 64-bit
      _STATISTICS_WRITING_DATE_UTC-eng: 2019-06-17 20:41:37
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
    Stream #0:1(ita): Audio: aac (LC), 48000 Hz, stereo, fltp (default)
    Metadata:
      BPS-eng         : 113000
      DURATION-eng    : 01:03:10.165000000
      NUMBER_OF_FRAMES-eng: 177664
      NUMBER_OF_BYTES-eng: 53536337
      _STATISTICS_WRITING_APP-eng: mkvmerge v30.1.0 ('Forever And More') 64-bit
      _STATISTICS_WRITING_DATE_UTC-eng: 2019-06-17 20:41:37
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
    Stream #0:2: Video: h264 (High), yuv420p(progressive), 720x404 [SAR 1:1 DAR 180:101], 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
    Metadata:
      BPS-eng         : 1100698
      DURATION-eng    : 01:03:10.080000000
      NUMBER_OF_FRAMES-eng: 94752
      NUMBER_OF_BYTES-eng: 521466768
      _STATISTICS_WRITING_APP-eng: mkvmerge v30.1.0 ('Forever And More') 64-bit
      _STATISTICS_WRITING_DATE_UTC-eng: 2019-06-17 20:41:37
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
Automatic encoder selection failed for output stream #0:2. Default encoder for format mp4 (codec none) is probably disabled. Please choose an encoder manually.
Error selecting an encoder for stream 0:2

This is my current command:

ffmpeg -progress pipe:1 -hide_banner -i /path/to/file.mkv -map 0:2 -map 0:1 -map 0:0 -c:a aac -c:v copy /path/to/outfile.mp4

Does anyone know how to do this using only one pass with FFmpeg?

Solution

You haven't set subtitle codec mode so ffmpeg will try to automatically select a subtitle encoder. However, the MP4 muxer hasn't registered a default subtitle encoder, so the command fails.

Use

ffmpeg -progress pipe:1 -hide_banner -i /path/to/file.mkv -map 0:v -map 0:a -map 0:s -c:a aac -c:v copy -c:s mov_text /path/to/outfile.mp4

Note that SRT is not officially supported in MP4s, so you have to re-encode it.