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 Debian possible default full paths are:

/usr/bin/who@ - from package: ucspi-tcp-ipv6
/usr/bin/who@ - from package: ucspi-tcp

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