usermod: 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/usermod - from package: shadow-utils

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

On Debian possible default full paths are:

/usr/sbin/usermod - from package: passwd

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

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