How to activate Apple product warranty on my MacBook Air?

You're not communicating with Apple when you post in the Community. Apple does not participate in these forums. We're all just fellow users, helping out as we have time and knowledge to share. 

As to your Mac, as Kilted Tim said, Apple does not have any authorized dealers in Dhaka, so the Mac you purchased would be considered to be "used" for the purposes of warranty. That means that the warranty has almost certainly expired by this time since the warranty would start when whoever was the actual authorized dealer first sold it.

Regards.

According to this: Find Locations - Apple (BD)

There are no authorized Apple retailers in that city.

It seems you may have purchased a gray market device.

The M1 MacBook Air was released in 2020 and was sold until earlier this year, so there is a high chance that your MacBook may have been manufactured over a year ago, meaning that the warranty has expired.

Sorry, but I don't know what you mean. What to you mean by "not covered"? What is it you're trying to do or get done?