users-admin: 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/users-admin - from package: gnome-system-tools

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

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