pdp9: 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/pdp9 - from package: simh

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

Error in other languages:
pdp9: bevel nie gevind nie
pdp9: comandă negăsită
pdp9: comando não encontrado
pdp9: commande introuvable
pdp9: command not found
pdp9: käsku ei ole
pdp9: không tìm thấy lệnh
pdp9: komanda nerasta
pdp9: Kommando nicht gefunden.
pdp9: kommandot finns inte
pdp9: komut yok
pdp9: nie znaleziono polecenia
pdp9: níor aimsíodh an t-ordú
pdp9: no se encontró la orden
pdp9: no s'ha trobat l'ordre
pdp9: opdracht niet gevonden
pdp9: parancs nem található
pdp9: perintah tidak ditemukan
pdp9: príkaz nenájdený
pdp9: příkaz nenalezen
pdp9: команда не найдена
pdp9: командата не е открита
pdp9: 命令找不到
pdp9: コマンドが見つかりません