diffindex-rred: 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 Debian possible default full paths are:

/usr/bin/diffindex-rred - from package: apt-file

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

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