@Bill1176 Just enter the nearest dates available and explain that you're flexible. If no dates are available, then there's not much you can do, and it's pretty pointless to message in that case anyway. It frustrates hosts too, the way Airbnb have this set up.
@Bill1176 Considering that inquiries are just a way for guests to ask questions without committing to a booking, it makes no sense to have to enter dates, but unfortunately you do.
So as advised, just enter whatever dates are available, and if the whole calendar is blocked, you're outta luck.