xpp_sync: 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/xpp_sync - from package: dahdi-tools

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

On Debian possible default full paths are:

/usr/sbin/xpp_sync - from package: dahdi

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

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