You are folding the front wheel back.
(If you manage it in this position you
are actually no beginner anymore, but nevertheless it is...

... wrong!
This was a bit too early for the front wheel has a hook at its left side. But this can not be hooked anywhere at present. Furthermore the Brompton in the picture above is not stable (only because it leans against the wall I could make the photo). While doing the right folding the Brompton stands on its own at every step).
Only possibility now:
Start again!