ftptop: 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/ftptop - from package: proftpd-utils

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

On Debian possible default full paths are:

/usr/bin/ftptop - from package: proftpd-basic

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

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