pkdiff: 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/pkdiff - from package: pktools

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

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