docdiff: 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/docdiff - from package: docdiff

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

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