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

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

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