pdp7: 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/pdp7 - from package: simh

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

Error in other languages:
pdp7: bevel nie gevind nie
pdp7: comandă negăsită
pdp7: comando não encontrado
pdp7: commande introuvable
pdp7: command not found
pdp7: käsku ei ole
pdp7: không tìm thấy lệnh
pdp7: komanda nerasta
pdp7: Kommando nicht gefunden.
pdp7: kommandot finns inte
pdp7: komut yok
pdp7: nie znaleziono polecenia
pdp7: níor aimsíodh an t-ordú
pdp7: no se encontró la orden
pdp7: no s'ha trobat l'ordre
pdp7: opdracht niet gevonden
pdp7: parancs nem található
pdp7: perintah tidak ditemukan
pdp7: príkaz nenájdený
pdp7: příkaz nenalezen
pdp7: команда не найдена
pdp7: командата не е открита
pdp7: 命令找不到
pdp7: コマンドが見つかりません