backportpackage: 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/backportpackage - from package: ubuntu-dev-tools

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

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