modules-used: 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/modules-used - from package: libmodule-used-perl

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

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