Welcome to UF.
That makes it sound like you are not the primary user of the machine, that is to say that you don't have admin privileges.
System will prompt you for a password, enter your regular login password - if it is rejected then you don't have admin privileges for this system and you need to talk to the person that does.
sudo nano /etc/xdg/autostart/gnome-screensaver.desktop
If it opens a text editor with content then look for
and change the 5 to an 8, press [CTRL]+[X] and type [Y], [Enter] - log out and log back into your desktop and open the 'screensaver' item off 'System'->'Preferences' again to check that it is changed to 8.