rwho: 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/rwho - from package: rwho

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

On Debian possible default full paths are:

/usr/bin/rwho - from package: rwho

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

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