pdp15: 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/pdp15 - from package: simh

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

Error in other languages:
pdp15: bevel nie gevind nie
pdp15: comandă negăsită
pdp15: comando não encontrado
pdp15: commande introuvable
pdp15: command not found
pdp15: käsku ei ole
pdp15: không tìm thấy lệnh
pdp15: komanda nerasta
pdp15: Kommando nicht gefunden.
pdp15: kommandot finns inte
pdp15: komut yok
pdp15: nie znaleziono polecenia
pdp15: níor aimsíodh an t-ordú
pdp15: no se encontró la orden
pdp15: no s'ha trobat l'ordre
pdp15: opdracht niet gevonden
pdp15: parancs nem található
pdp15: perintah tidak ditemukan
pdp15: príkaz nenájdený
pdp15: příkaz nenalezen
pdp15: команда не найдена
pdp15: командата не е открита
pdp15: 命令找不到
pdp15: コマンドが見つかりません