I have installed Manjaro Cinamon. How can I enable now full disk encryption? Does Manjaro provide some GUI for this?
As far as I'm aware of I would say NO. But if you wanted a full disk encryption then you should have tried LUKS while installing the system. That provides a key encrypted disk or drive which can only be accessed using the Key.
In case you forget the key then the only way out of it would be reinstalling everything.
You can most probably reinstall the system (but do make a proper backup of all the files) or you can follow the following tutorial
I'm providing you the links from where you can understand about LUKS a bit more
And do tell me if you find a way to encrypt the system using GUI interface.