latexdiff: 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/latexdiff - from package: latexdiff

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

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