Why i can’t download the app YouTube in my country ?

Not all apps are available in all countries. Sometimes they are prohibited by the country, at other times the developer decides not to distribute in that country. You can contact the app developer and ask:

Click on this link to discover 'How to contact an app developer' - https://support.apple.com/HT207959

Availability of Apple Media Services - https://support.apple.com/HT204411 - "Learn if you can buy apps, movies, music, books, and more, or use Apple Podcasts or Apple News, in your country or region."

What country do you live in? You can Google Countries where YouTube is banned and there is a list of at least 10 countries. If you live in one of them, that is why you can't download the app.