What this screen is
What the QR code on the clubhouse wall opens. It tells the desk you have arrived.
Why scanning it does not check you in
Visiting the address checks nobody in. It opens a screen that asks who you are and makes you press something - which is the whole reason the poster is safe to tape up. A link that checked people in would check in whoever photographed it.
If your name is not on the list
You are looking at the right competition but you are not entered in it, or you are entered under a different spelling. Speak to the desk; they can check you in themselves.
If you checked in by mistake
Tell the desk. They can undo it, and the change is recorded.