texdiff: 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/texdiff - from package: texlive-extra-utils

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

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