avformat_write_header return error code when trying to write video with G711U audio avi file

I'm trying to make an avi file from a video/audio stream. When audio is AAC it works fine. But I cannot pack G711U audio.

  av_register_all();
  avcodec_register_all();
  av_log_set_callback(nullptr);

  av_format_context_ = avformat_alloc_context();
  if (!av_format_context_)
  {
    std::string error_msg = "Failed to allocate avformat context";
    log_->Error(error_msg);
    throw std::runtime_error(error_msg.c_str());
  }

  av_format_context_->oformat = av_guess_format("mp4", filename.c_str(), NULL);
  strcpy(av_format_context_->filename, filename.c_str());

  AVCodec* codec = avcodec_find_encoder(AV_CODEC_ID_PCM_MULAW);
  av_format_context_->oformat->audio_codec = codec->id;

  audio_stream_ = avformat_new_stream(av_format_context_, codec);
  if (audio_stream_ == nullptr)
  {
    log_->Error("Can not create audiostream.");
    return false;
  }

  AVCodecContext& codec_context = *audio_stream_->codec;
  if (avcodec_copy_context(audio_stream_->codec, av_format_context_->streams[1]->codec) != 0)
  {
    log_->Error("Failed to Copy Context");
    return false;
  }

  audio_stream_->time_base = { 1, codec_context.sample_rate };

  if (av_format_context_->oformat->flags & AVFMT_GLOBALHEADER)
    codec_context.flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

  log_->Info("Init output {}", filename);
  av_dump_format(av_format_context_, 0, filename.c_str(), 1);

  int ret = avio_open(&av_format_context_->pb, filename.c_str(), AVIO_FLAG_WRITE);
  if (ret < 0)
  {
    log_->Error("Can not open file for writing.");
    return false;
  }

  ret = avformat_write_header(av_format_context_, NULL);
  if (ret < 0)
  {
    log_->Error("Can not write header.");
    return false;
  }

Here avformat_write_header() returns -22. What's wrong here? Which data should I supply to this function?

In your code you tried to create MP4 container format output file instead AVI: when you've provided short name in av_guess_format as first parameter, it has more weight for decide output format than file name extension (https://www.ffmpeg.org/doxygen/0.6/libavformat_2utils_8c-source.html#l00198).

MP4 container does not support PCM data including G.711. Please look on this page for details https://en.wikipedia.org/wiki/Comparison_of_video_container_formats