diffpp: 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/bin/diffpp - from package: enscript

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

On Debian possible default full paths are:

/usr/bin/diffpp - from package: enscript

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

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