ipppstats: 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/ipppstats - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/sbin/ipppstats - from package: ipppd

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

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