demo.php: 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/share/simplesamlphp/modules/oauth/bin/demo.php - from package: simplesamlphp

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

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