users: 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/bin/users - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/users - from package: coreutils

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

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