Installtion of erpnext

hello , I am trying to install erp. but I am getting error please help me .

error >>>>

:~/frappe-bench$ bench install-app erpnext
Installing erpnext…

ERPNext can only be installed on a fresh site where the setup wizard is not completed
You can reinstall this site (after saving your data) using: bench --site [sitename] reinstall

Restart setup on the fresh system without any preinstalled packages such as redis, mysql server, niginx, supervisor etc.

use and follow simple two steps to complete automated installation.