who: 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/who - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/who - from package: coreutils

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

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