pdp11: 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/pdp11 - from package: simh

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

Error in other languages:
pdp11: bevel nie gevind nie
pdp11: comandă negăsită
pdp11: comando não encontrado
pdp11: commande introuvable
pdp11: command not found
pdp11: käsku ei ole
pdp11: không tìm thấy lệnh
pdp11: komanda nerasta
pdp11: Kommando nicht gefunden.
pdp11: kommandot finns inte
pdp11: komut yok
pdp11: nie znaleziono polecenia
pdp11: níor aimsíodh an t-ordú
pdp11: no se encontró la orden
pdp11: no s'ha trobat l'ordre
pdp11: opdracht niet gevonden
pdp11: parancs nem található
pdp11: perintah tidak ditemukan
pdp11: príkaz nenájdený
pdp11: příkaz nenalezen
pdp11: команда не найдена
pdp11: командата не е открита
pdp11: 命令找不到
pdp11: コマンドが見つかりません