ruptime: 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/ruptime - from package: rwho

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

On Debian possible default full paths are:

/usr/bin/ruptime - from package: rwho

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

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