warnquota: 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/warnquota - from package: quota

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

On Debian possible default full paths are:

/usr/sbin/warnquota - from package: quota

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

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