Question Error with hostname when installing Plesk

[l0nglive]

 Checking for the user 'mhandlers-user'...
 User 'mhandlers-user' already exists

 Trying to got legacy variables... done
 Trying to save legacy variables... done
Warning!
Unable to detect the fully qualified domain name of the host.
Please make sure that FQDN of the host is correctly specified in /etc/hosts and goes right after the host's IP address
(depending on your OS, you might need to set the host name in /etc/HOSTNAME or /etc/hostname).

Setting it to localhost.localdomain
 Trying to resolve hostname 'localhost.localdomain' and validate its IP address...
Unable to resolve host name.
Please specify the host name in the /etc/hosts file or specify DNS configuration in the /etc/resolv.conf on your system.

ERROR while trying to check the hostname
Check the error reason(see log file: /var/log/plesk/install/plesk_12.5.30_installation.log), fix and try again

*****  problem report *****
ERROR while trying to check the hostname
Check the error reason(see log file: /var/log/plesk/install/plesk_12.5.30_installation.log), fix and try again

ERROR: An error occurred during performing of installation PREP actions (see log for details). Installation was rolled back.
Warning! Not all packages were installed.
Please check if your system services are operable.
Please resolve this issue and try installing the packages again.
If you cannot resolve the issue on your own, contact product technical support for assistance.

I have this problem with hostname. Doesn't anyone know how to edit my hostfile correctly?
my VPS IP is 163.172.178.215