xtrabackup: 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 Debian possible default full paths are:

/usr/bin/xtrabackup - from package: percona-xtrabackup

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

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