xxdiff: 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/xxdiff - from package: xxdiff

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

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