mkfs.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/mkfs.btrfs - from package: btrfs-progs

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

On Debian possible default full paths are:

/sbin/mkfs.btrfs - from package: btrfs-tools

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

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