adduser: 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/adduser - from package: shadow-utils

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

On Debian possible default full paths are:

/usr/sbin/adduser - from package: adduser

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

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