idevicebackup2: 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/idevicebackup2 - from package: libimobiledevice-utils

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

Error in other languages:
idevicebackup2: bevel nie gevind nie
idevicebackup2: comandă negăsită
idevicebackup2: comando não encontrado
idevicebackup2: commande introuvable
idevicebackup2: command not found
idevicebackup2: käsku ei ole
idevicebackup2: không tìm thấy lệnh
idevicebackup2: komanda nerasta
idevicebackup2: Kommando nicht gefunden.
idevicebackup2: kommandot finns inte
idevicebackup2: komut yok
idevicebackup2: nie znaleziono polecenia
idevicebackup2: níor aimsíodh an t-ordú
idevicebackup2: no se encontró la orden
idevicebackup2: no s'ha trobat l'ordre
idevicebackup2: opdracht niet gevonden
idevicebackup2: parancs nem található
idevicebackup2: perintah tidak ditemukan
idevicebackup2: príkaz nenájdený
idevicebackup2: příkaz nenalezen
idevicebackup2: команда не найдена
idevicebackup2: командата не е открита
idevicebackup2: 命令找不到
idevicebackup2: コマンドが見つかりません