regdiff: 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 CentOS possible default full paths are:

/usr/bin/regdiff - from package: samba4-client

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/regdiff - from package: registry-tools

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

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