Did you mess around with your pacman config and accidentally deleted the entry for the core repository? The grub package is inside the core repo. There’s no way the package couldn’t be found.
There should be the following entry inside your /etc/pacman.conf:
[core]
Include = /etc/pacman.d/mirrorlist
Does pacman -Ss grub and pacman -Si grub return any results? What about pacman -Qi grub?
And I’m actually a bit confused here. Were you using grub before this? If you are, how can the grub-mkconfig be missing? Did you uninstall the package? Please provide more information. Because at this point, I don’t really know what to make of your situation except that something is very, very wrong with your system.