Hey there, Хасан Хасанов
Unfortunately, you have hit the store at the wrong time to try and purchase Minecraft. At the moment, because Bulgaria is transitioning from Bulgarian currency to Euro, all purchases have been suspended. You can find out more about this in this learn article here: https://learn.microsoft.com/en-us/windows/apps/publish/bulgarian-currency
At the moment, they are hoping this is restored early January, but will be completed fully by the end of February. From what I can make out, Microsoft is going to be displaying messages on effected stores so you are aware. While this is happening, it may be worth seeing if you can purchase Minecraft from a physical store in your region.
Hope this is helpful & Happy New Year!
Nathan