Export multichannel FLAC file

Hello friends, I accidentally found a multichannel MP4 5.1 video on YouTube. I can import it into Audacity without any problems, and then I see the 6 channels as well.
I then set several text markers and now wanted to export these individual parts into multichannel FLAC files. Unfortunately, that doesn’t work, or I don’t know how.
Can someone please help me?
Warm greetings
AudioFan

To export multi-channel audio files, enable “Advanced Mixing Options” in Preferences (see: https://manual.audacityteam.org/man/import_export_preferences.html). When that’s enabled, the “Advanced Mixing Options” will appear when exporting. See: https://manual.audacityteam.org/man/advanced_mixing_options.html