Welcome!
it's great you joined the Android Community
apparently, i think your phone can't take screenshots with its default settings but you can download applications like this one
https://play.google.com/store/apps/...0uYW5kcm9pZC5zY3JlZW5zaG90aXRmdWxsbm9yb290Il0. (paid version though but you don't need root for this app)
Phones like the Galaxy Nexus, part of the Google Nexus series, are designed to allow root access[1]
Rooting is a process allowing users of smartphones, tablets, and other devices running the Android operating system to attain privileged control (known as "root access") within Android's subsystem. Rooting is often performed with the goal of overcoming limitations that carriers and hardware manufacturers put on some devices, resulting in the ability to alter or replace system applications and settings, run specialized apps that require administrator-level permissions, or perform other operations that are otherwise inaccessible to a normal Android user. Rooting is analogous to jailbreaking devices running the Apple iOS operating system or the Sony PlayStation 3. On Android, rooting can also facilitate the complete removal and replacement of the device's operating system. -Wikipedia
Rooting's when you get all the access you need for your phone so you can fully tweak your phone
-if this helped, hit thanks please