When you are using an efi system - as your partition layout suggests - you should use something like this for a Manjaro default system
grub-install --target=x86_64-efi --efi-directory=/boot/efi --recheck --bootloader-id=Manjaro
And subsequently
grub-mkconfig -o /boot/grub/grub.cfg
Due to several topics raising the question I have created a short guide aimed at correctly identifying which method is applicable for a given system.