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.