BTRFS: File system unmountable, no boot and access possible

General system Newbie
13

I am on live media. The device is not mounted anywhere:

[liveuser@eos-2021.12.17 ~]$ sudo umount /mnt
umount: /mnt: not mounted.
[liveuser@eos-2021.12.17 ~]$ sudo mount -t btrfs /dev/nvme0n1p2 /mnt -o subvol=@
mount: /mnt: mount(2) system call failed: File exists.
[liveuser@eos-2021.12.17 ~]$