you boot on USB iso manjaro
open a terminal
before chroot
inxi -Mza ( check this is UEFI only )
sudo efibootmgr -v
test -d /sys/firmware/efi && echo efi || echo bios
if ok
sudo manjaro-chroot -a
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck --verbose
mkinitcpio -P
update-grub
efibootmgr -v
exit ( quit chroot )
then reboot