pppstats: command not found

Have You tried accessing the binary with the full path?

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/pppstats - from package: ppp

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

On Debian possible default full paths are:

/usr/sbin/pppstats - from package: ppp

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