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[megavolt]

You are lucky, the partition scheme is GPT:

@a_0vi

  ID-1: /dev/nvme0n1 maj-min: 259:0 vendor: Intel model: SSDPEKKW256G7
    size: 238.47 GiB block-size: physical: 512 B logical: 512 B speed: 31.6 Gb/s
    lanes: 4 tech: SSD serial: <filter> fw-rev: PSF121C temp: 17.9 C
    scheme: GPT

Keep sure you create a 100-300MB vfat partition flagged as efi partition and mount it under /mnt/boot/efi. See what chroot is:

@system

3. manjaro-chroot - auto

Boot up from live Manjaro media then enter into the terminal:

manjaro-chroot -a

This is automatic chroot, useful when multi boot, will list all bootable partitions, select the proper (Manjaro) root partition then proceed with updating or fixing the system

4. manjaro-chroot

For example on the right I used sda1 is / manjaro; sda2 is swap, sda3 is home

lsblk ← list partitions
sudo mount /dev/sdxx /mnt ← example: sudo mount /dev/sda1 /mnt
sudo mount /dev/sdxx /mnt/home/ ← example: sudo mount /dev/sda3 /mnt/home/
sudo mount /mnt/boot/efi ← (optional) if you have UEFI!
sudo manjaro-chroot /mnt

Now you can start updating/fixing your system…

You would need to completely remove /mnt/boot/grub/ and then reinstall grub that way: GRUB/Restore the GRUB Bootloader - Manjaro