proftpd: 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/sbin/proftpd - from package: proftpd

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

On Debian possible default full paths are:

/usr/sbin/proftpd - from package: proftpd-basic

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

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