Debian btrfs filesystem shows a total of zero inodes total, zero used, zero free
As a modern filesystem, btrfs
has no fixed inode limit at all, and that's why it reports inodes as all zeroes.
Check the status of btrfs
subvolumes:
btrfs subvolume list -s /
If it turns out that you have snapshots hogging your disk space, you might need something like this to remove them:
btrfs subvolume delete -c /.snapshots/NNN/snapshot
See also this link for another user's adventure with btrfs and snapshots. The comments on that webpage include useful btrfs
management commands among all the salt.