That is not the correct way to mount a BTRFS sub-volume…
You should start by mounting your root sub-volume @ at /mnt/manjaro-root
If /dev/sda4 is your ESP you would continue with mounting that at /mnt/manjaro-root/boot/efi
Followed by chroot as sudo manjaro-chroot /mnt/manjaro-root (no @ at end)
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