I am not able to download prime video. when I try through website it says it is not available in my region ?
It is up to the channel developer to make a channel available to your region. Same with your other post. You should contact the developer.
Are you saying that the app is not available in India?
If that is what the Roku Channel Store is saying, then yes. Amazon has not made it available for your region. You would need to contact them.