Why game pass is not available in my region?

Do you have Game Pass? It's available via Game Pass for Consoles, so if you do, you should be able to find it ready to install in My Games & Apps under the Game Pass Subscription section.

I'd also verify that Life is Strange is available in your region. Not all titles are available in every region, which might be why you're unable to download or purchase the title.

If that is the case, Xbox Live appears to be available in your region. At this time, Game Pass is not. That may be due to governmental regulations or a perceived lack of interest in the service in the market. As Brendan stated, they may well be working
to get the service to your region, and when it's available, it will be publicized.

Bulgaria, as I mentioned my neighbor countries, Greece and Turkey, has it. Most of fellow European Union counties has it.

@ngochpi thats the entire problem, it doesn't exist in the menu and there is no such an option like 'other' or 'international'

So, there is no problem using free month keys coming with the xbox, but is a problem when i want to pay for more?

Microsoft is still working on spreading the availability of Xbox Live and Game Pass. Game Pass in particular is more difficult to take international because they most likely have to get separate licenses for each game for many (if not all) of the regions
they choose to operate in. I'm sure there are regions that will never get Game Pass because the cost to offer it won't be made up by the amount of accounts opened.