ftpwho: 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/ftpwho - from package: proftpd-utils

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

On Debian possible default full paths are:

/usr/bin/ftpwho - from package: muddleftpd
/usr/bin/ftpwho - from package: proftpd-basic

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

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