Normally, you would use manjaro-chroot, which will autodetect your EFI partition and mount it in the right place. The error message you’re showing says that the mountpoint does not exist ─ a mountpoint is a directory ─ and it is unclear whether you are doing this from within a chroot environment or not.
From within the live environment, issue the following two commands… 
sudo su -
manjaro-chroot -a
If you are presented with a choice, select your Manjaro root partition. Then reinstall GRUB.
grub-install --recheck --no-rs-codes --modules="part_msdos part_gpt" --target="x86_64-efi" /dev/sda
update-grub