adsl-stop: 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/adsl-stop - from package: rp-pppoe

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

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