Did you click the view details button? That is what it perhaps looks like. Go ahead and click the Start your build and see what you get. The Pro is definitely sold out from the last wave, the other 3 models should all have availability and let you order.
Contrary to a concert where there is a limited number of seats at a certain price, Ford hasn't built/delivered a single Lightning pro yet. How can something that hasn't been built yet, not be available anymore...
Just like you buy that seat to the concert a few months before the music begins to play. Some seats may no longer be available, so you'll opt for something else. Then you all still show up later to hear the music together.