Well, V16 Pro has 4 Editions; the Basic and Mechanical editions come with 2 Door/Window "Types" each, while the Architectural and Platinum editions has everything (20Â±?). If you click on the Double Door value, it should show you a list of door types; either Single and Double-Dhung or a whole series of doors (depending on your edition).
When you have the "Standard" style selected in Door Styles, you should be able to click on a "New" icon to create a new style.
You can open your V14 files in V16, but I would recommend doing this with "copies" of those V14 files until you've become accustomed to V16.
Are you using "New From Scratch" for a new drawing or a Template? Looking at the Architectural Template in Deluxe, I noticed that it comes with a Double door in one of the supplied styles, while that style is not one of the defaults. A little present for customers.
Designer, Deluxe, (Basic, Expert, Professional), Platinum
Windows 10 Pro (1909), 64-bit