Mkv hasn't been a format used much in professional editing, though heavily used in som niche segments such as historical collections and archivists.
I've seen a couple posts here where someone said they were now able to import and work with mkv files, but I have no such media to test.
You can make mkv in Handbrake....
now I understand
but even if mkv is not professional I think the premiere should give support since it is a professional program so it should work with every kind of file
That would require more budget than the company seems willing to invest in that issue. There are many of the smaller-used formats/codecs that are very useful in certain areas but not supported within PrPro. Personally, between increasing engineering time on support for more niche formats, and increasing stability with the main used formats ... well, I hold with the second option.
Please understand ... I would be fine with mkv support in PrPro. It is a good quality format, and for some things very useful. Yet there are some far more common pro formats that have issues in certain uses or just plain troubles in PrPro. Those need sorting before adding full to my train of thought, unless mkv support takes very little attention.
Just a priorities issue.
As of 7/23/2020, mkv container format is also not supported in the Adobe non-professional video editing product - Adobe Premier Elements - which is specifically geared towards hobbyists. My guess is due to a shared code base between the products.
There is now limited, undocumented support for mkv. As of PR 2018.1.1.
See this thread. Post 61 and after is where this change is discussed.
For me, audio format was one variable that would result in no import.
I made a 4K 50p H.265 mkv in Handbrake and it plays well in Pr.
I am abandoning Adobe. All the videos on youtube are now MKV and yet Adobe doesn't support MKV. What the hell do I pay a subscription for! At least enable Media Encoder to convert the stupid thing! Instead here I am paying a subscription fee and scrounging online like a loser looking for something that will work and subscribing to other things. Stupid!!
All videos on YouTube are mkv?
Not a format I've heard they transcode to. Weird.
OP is probably using a downloader that makes the movies into mkv.
If one is that eager to download from YT to use in Pr, use a downloader that makes mp4.