The signal message is clear: your handler method onArea2Dinputevent expects one parameter, while it is called with three (through the signal connection). Looking at the docs for the area2d input event signal, I can see the expected signature is:
_on_Area2D_input_event( viewport, event, shape_idx )
Regarding the curly braces '{', if you are referring to typing them, that depends on your keyboard layout; on my (Italian) keyboard, the key combination is Shift + whatever you do to type '[' .
You are already aware that you need to clean up roll calls. Notice that class variables mention above are fit for representing some persistent "state" of a class, like life points of a character. Since rolling a dice is a transient result which changes every time ("stateless"), rolladice is more aptly refactored as a simple function, returning a value without setting anything external:
func roll_a_dice(minimum, maximum):
return randi() % (maximum-minimum+1) + minimum
and then call inside the event handler.