TOTO ZABRALO

sudo service lightdm stop

sudo service gdm stop

sudo service kdm stop //this is the one that worked for mi as I use kdm and Linux mint

As the error states, you are still running an X server. This error occurs when you try to install the Nvidia .run files while logged in.

Make sure you are logged out.

Hit CTRL+ALT+F1 and login using your credentials.
kill your current X server session by typing sudo service lightdm stop or sudo stop lightdm
Enter runlevel 3 (or 5) by typing sudo init 3 (or sudo init 5) and install your .run file.
You might be required to reboot when the installation finishes. If not, run sudo service lightdm start or sudo start lightdm to start your X server again.