btrfs-image: 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-image - from package: btrfs-progs

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

On Debian possible default full paths are:

/sbin/btrfs-image - from package: btrfs-tools

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

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