ftpquota: 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/ftpquota - from package: proftpd-utils

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

On Debian possible default full paths are:

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

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

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