No, I don't believe there is nothing that can LEGALLY stop you jumping in and going for a flight.
However, your fellow group members might be a little chavved off when you bent it on landing/takeoff. Also, it is unlikely that the insurance company would cough up when they discovered your lack of tailwheel experience.
A tailwheel conversion takes as little as 3 hours.
I suggest you keep sitting in it making engine noises until you've been checked out