Set board manager as ATmega16
Thanks for everyone help here.

I have resolved by few steps.

1) File -> preference -> Board manager URL as like below. Basically URL are seperated by commas.
http://arduino.esp8266.com/stable/package_esp8266com_index.json,https://mcudude.github.io/MightyCore/package_MCUdude_MightyCore_index.json

2) Deleted the tmp file (package_index.json.tmp) which is in the path C:\Users\SIMBU\AppData\Local\Arduino15\ 

This helped me to resolve the issue.