Does not support mkv videos from youtube
YouTube doesn't offer videos in mkv containers at all. YouTube offers mp4 and webm, which is similar to mkv. From video codecs YouTube uses in its webm videos, Avidemux supports vp8 and vp9 for decoding. It doesn't support AV1, in case you have got such a video.
does not support mkv videos with opus audio,I get an error when dragging it to the avidemux converter https://imgur.com/a/2IJDHwX
https://imgur.com/a/Pm8UEf5 this is error dont no open
Works fine for me using streams 247 and 251 for yt-id g_bNF71v9ek regarding loading the webm video. Please attach admlog.txt from the attempt to open the video and provide the file as a sample using WeTransfer or a similar service.
Saving in copy mode didn't work, but should be fixed now -->  (https://github.com/mean00/avidemux2/commit/1eb4addae0f1260d0a0805d8ba54793018ec69fa) +  (https://github.com/mean00/avidemux2/commit/ffc5718b9fe5ed08ae567718d63dbf4919c2ba78), fix effective in future nightlies.
https://www.youtube.com/watch?v=brg2aRN2URM&feature=youtu.be this is demo what no work
Saving in copy mode will work with future nightlies, it can't work now as no nightlies with the changesets mentioned above have been generated yet. Loading a .webm of -rnTb-dR7lM containing 247+251 works fine for me. We won't advance as long as you don't provide more information.
So please, please...
1) use the latest nightly
2) reproduce the failure to load the file and attach admlog.txt from that Avidemux session
3) provide the file as a sample
Regarding your edited yt-URL, this video is unavailable at my location at least.
https://drive.google.com/file/d/1iE2rPRxcSbtRK1Qi6fj8pl8te-crQyCd/view this sample with 2.7.3 release
https://drive.google.com/file/d/1YxrGFYYu0yG1A06NmzV5YCRNJIeQMMqz/view this sample 2.73 nightly
this is admlog
From the Avidemux log, it looks like the MKV file you try to load is malformed (broken) or for other reasons unsupported. It is impossible to verify that because you haven't provided it as a sample yet (providing screencasts makes no sense).
What do you use to obtain YT videos you mentioned? youtube-dl paired with an up-to-date ffmpeg works just fine to mux individual streams to valid MKV files.
yes i do mkv another archive
I have not a slightest idea what "another archive" you are using and in what particular way the files from that source are broken or not broken (i.e. whether the bug is on their part or on our). The usual way (youtube-dl + ffmpeg) works. It is up to you to provide necessary input to allow to assess the problem you experience when sticking with a different source.