latexdiff-fast: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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-fast - from package: latexdiff

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