HELP WITH PERMISION DENIED ERROR WHEN DOING /dev/mapper/root / ext4 defaults 0 2

Good morning, I am remotely luks encrypting an Ubuntu 18.04 dedicated server that will have remote unlocking via dropbear ssh at reboot time to decrypt the disk. I have installed dropbear and busybox first and setup and configured them, next I have booted a recovery image and logged in. I've made a copy of root partition luks encrypted the old one and so on and created a new file system. The problem comes when I change root to the new root, and then I try to update /etc/fstab to work with the new file system by running the command /dev/mapper/root / ext4 defaults 0 2 it keeps giving me the following permission's error: bash: /dev/mapper/root: Permission denied .