I am trying to create a sitemap file in configuration folder, but the permission is denied! How to overcome it and change permissions of a folder?
Assuming you have a linux based, apt-get install system, quick and dirty solution (gives everyone read/write):
sudo chmod 777 -R /etc/openhab2/
I would however recommend reading:
http://docs.openhab.org/installation/index.html
The section applying to your OS
my directory is /opt/openhab
so code will be sudo chmod 777 -R /opt/opehab
am i right?
ignoring the typos, yes 
But the thing is after changing sitemap with permission granted openhab is not working 
Giving these rights to your config folder should not have any influence on your system as such.
What exactly do you mean by “not working”?