Thanks for reporting.
MP4 opus audio stream failure - Caused by the attempt to mux the downloaded audio codec opus into an MP4 container. MP4 does not support opus so extra conversion is required.
WEBM VP9 video stream failure - VP9 is experimental in version of ffmpeg we distribute and is failing to mux.
Will handle this in the next release.
In the mean time select an MP4 H264/AAC stream and this will download and mux correctly. This is most commonly what appears first in the list of available streams from youtube.
From CRS @ 03 Oct 2019 08:10