On ArchLinux or any other Linux based distribution you get an error like
error: you cannot perform this operation unless you are root.
Or something similar, it means the command you're running need "root" permission which will make some system wide changes and it's asking you for something like permissions.
Simply run your command with command sudo as prefix.
For example running:
pacman -Syu firefox
sudo pacman -Syu firefox
That's it ;)