Please help me unlock my iPhone 6

Your phone is locked to a different carrier, you will need to contact the carrier it is locked to and ask them to unlock it before you can use it with your new carrier.

How to unlock your iPhone for use with a different carrier

https://support.apple.com/en-us/HT201328

Also, an iPhone 6 is a very old model running a very old operating system. You will find that most apps won't run on iOS 12.5.4. You may want to return it and purchase a newer model that is capable of running the latest iOS.

No. We don't know. You can call AppleCare and give them the serial number. They might be able to tell you.

By the way, telling us it's difficult in your country is pretty ineffective when we don't know what country you're in.

It doesn't matter where you live, you need to unlock you phone with the previous carrier. It's not going to work until you do.

Here

https://www.google.com/m?q=Belarus.&client=ms-opera-mobile&channel=new&espv=1