I was able to fix this thanks to the other comments here!
SOLVE:
In the Categories window if the "Enable Page" is greyed out try clicking: "ADD TO YOUR MAIN NAVIGATION" (at the bottom of the window) and make sure both Visibility and Enable Page are toggled on. All my categories showed up right away.
Let me know if this works for others!
I am struggling stylizing my main nav drop down menus and would greatly appreciate some help! Attached is the styling I am hoping to achieve. I have the arrow on the right, but I want it to turn upside down on hover as well. TIA