ppp-watch: 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:

/sbin/ppp-watch - from package: initscripts

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

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