Use this link:
http://codex.wordpress.org/Create_A_Network
It will show you how to enable wordpress multisite.
It sounds like a issue with permissions. Depending on who your web host is, you might have to have them change the permissions on your website.
EDIT: It looks like you are running a outdated version of wordpress. You should probably update to 4.3.1.
Updating WordPress Codex Page: https://codex.wordpress.org/Updating_WordPress
