add.modules: 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 CentOS possible default full paths are:

/usr/share/Modules/bin/add.modules - from package: environment-modules

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/add.modules - from package: environment-modules

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

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