in.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/in.proftpd - from package: proftpd

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

On Debian possible default full paths are:

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

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

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