You must chroot your System
and then this in Your Terminal:
https://forum.manjaro.org/t/howto-chroot-from-or-into-any-linux-distribution/34071
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck
grub-mkconfig -o /boot/grub/grub.cfg