groupmod: 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/sbin/groupmod - from package: shadow-utils

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

On Debian possible default full paths are:

/usr/sbin/groupmod - from package: passwd

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

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