I have downloaded mac OS Sierra. Installation is not happening. I am trying for two days. What to do?

Is the installer running and not completing? Do you see Install Mac OS in your Applications folder? If you double click on it in the Applications folder, will it run?

Try setting up another admin user account to see if the same problem continues. Also try the Safe Mode.

Isolating an issue by using another user account

Safe Mode - About

Safe Mode Startup – El Capitan. also Sierra

You are welcome.