App Preview submission, video has audio that is not two-channel

For most apps that do not have the primary purpose of handling audio, I find background music or audio voice over to be a distraction in an app preview. If you want to add silence to preview, you can easily do it with ffmpeg:

ffmpeg -f lavfi -i anullsrc=channel_layout=stereo:sample_rate=44100 -i preview.mp4 -shortest -c:v copy -c:a aac previewWithSilence.mp4

If you're using iMovie:

1. Drag in any audio clip. It doesn't matter what it is.
2. Shorten the length to less than your project's total length.
3. Set the sound level to 0% by dragging the horizontal bar over the clip down.
4. Share as App Preview.

Unfortunately "no audio channel", isn't an option in the official docs.

I'd follow the official Apple guide over here and add some "background music":

Create a Soundtrack

1. Choose File > Import Media, then select the audio file in the window that appears.
2. Drag music to the Background Music area at the bottom of the timeline.
3. Drag sound effects directly underneath any clip in the timeline.

Apple is quite picky when it comes to the allowed file formats and configurations, but once you fixed that, it should work!