btrfs-show-super: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/sbin/btrfs-show-super - from package: btrfs-tools

You may install the required package with command apt-get install {package}
Error in other languages:
btrfs-show-super: bevel nie gevind nie
btrfs-show-super: comandă negăsită
btrfs-show-super: comando não encontrado
btrfs-show-super: commande introuvable
btrfs-show-super: command not found
btrfs-show-super: käsku ei ole
btrfs-show-super: không tìm thấy lệnh
btrfs-show-super: komanda nerasta
btrfs-show-super: Kommando nicht gefunden.
btrfs-show-super: kommandot finns inte
btrfs-show-super: komut yok
btrfs-show-super: nie znaleziono polecenia
btrfs-show-super: níor aimsíodh an t-ordú
btrfs-show-super: no se encontró la orden
btrfs-show-super: no s'ha trobat l'ordre
btrfs-show-super: opdracht niet gevonden
btrfs-show-super: parancs nem található
btrfs-show-super: perintah tidak ditemukan
btrfs-show-super: príkaz nenájdený
btrfs-show-super: příkaz nenalezen
btrfs-show-super: команда не найдена
btrfs-show-super: командата не е открита
btrfs-show-super: 命令找不到
btrfs-show-super: コマンドが見つかりません