btrfs: command not found

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 CentOS possible default full paths are:

/sbin/btrfs - from package: btrfs-progs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/sbin/btrfs - from package: btrfs-tools

You may install the required package with command apt-get install {package}

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