grepdiff: 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/grepdiff - from package: patchutils

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

On Debian possible default full paths are:

/usr/bin/grepdiff - from package: patchutils

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

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