install-postfix.sh: 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/share/gforge/bin/install-postfix.sh - from package: gforge-mta-postfix

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

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