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

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

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