pdp1: 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/pdp1 - from package: simh

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

Error in other languages:
pdp1: bevel nie gevind nie
pdp1: comandă negăsită
pdp1: comando não encontrado
pdp1: commande introuvable
pdp1: command not found
pdp1: käsku ei ole
pdp1: không tìm thấy lệnh
pdp1: komanda nerasta
pdp1: Kommando nicht gefunden.
pdp1: kommandot finns inte
pdp1: komut yok
pdp1: nie znaleziono polecenia
pdp1: níor aimsíodh an t-ordú
pdp1: no se encontró la orden
pdp1: no s'ha trobat l'ordre
pdp1: opdracht niet gevonden
pdp1: parancs nem található
pdp1: perintah tidak ditemukan
pdp1: príkaz nenájdený
pdp1: příkaz nenalezen
pdp1: команда не найдена
pdp1: командата не е открита
pdp1: 命令找不到
pdp1: コマンドが見つかりません