pure-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/pure-ftpwho - from package: pure-ftpd
/usr/sbin/pure-ftpwho - from package: pure-ftpd

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

On Debian possible default full paths are:

/usr/sbin/pure-ftpwho - from package: pure-ftpd-ldap
/usr/sbin/pure-ftpwho - from package: pure-ftpd-mysql
/usr/sbin/pure-ftpwho - from package: pure-ftpd-postgresql
/usr/sbin/pure-ftpwho - from package: pure-ftpd

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

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