I'm putting together a new desktop using a Samsung SM951 NVMe drive. I
booted sysrescd, partitioned the drive and ran

mkfs.btrfs /dev/nvme0n1p3
mount /dev/nvme0n1p3 /mnt/custom

df -T, mount and findmnt all show this is mounted as a btrfs filesystem,
e.g.

/dev/nvme0n1p3 on /mnt/custom type btrfs (rw,relatime,ssd,space_cache)

I can create files in here but cannot do anything btrfs-y

% btrfs filesystem show /mnt/custom
ERROR: not a valid btrfs filesystem: /mnt/custom

% btrfs subvolume create /mnt/custom/test
Create subvolume '/mnt/custom/test'
ERROR: cannot create subvolume: Inappropriate ioctl for device

Google has been no help at all.