HBO MAX

I don't have any problem locating the app HBO Max in the iOS App Store on my iPhone.

I am in the US. What country do you live in?

If HBO does not offer the app in your country, then you may not install it on your iPhone.

M1 Mac mini, macOS 11.4; Watch, series 5 GPS/cellular, watchOS 7.5; iPad Pro 2nd Gen, iPadOS 14.6; iPad Air 2, iPadOS 14.6; iPhone 12 Pro, iOS 14.6; TV 4th Gen, tvOS 14.6  HomePods, audioOS 14.6 and Airport Express w/ Airplay 2

Apple does not currently do consumer business in Bangladesh. There is no iTunes Store, nor is there an App Store for Bangladesh. You have taken an Apple product outside of Apple's business market. There is no way for you to legally download apps for an iPhone in Bangladesh.