You are probably running into some of the unique "features" of the Frontier website. You usually can't see awards unless you have the points. I also ran into my usual issue of the flight search giving weird results on the first try. It was telling me that the city pairs were not eligible, but re-running the search from the error page gave me results.
I looked at December 9, return Dec. 16 (Tuesdays) and got a morning flight out of LGA and two evening returns from LAS at 15K round-trip. Clicked through to the confirmation page to see if they were valid and they came up fine with $10 taxes total.
How does Amex give you the points? Do they just transfer 15K into Early Returns? If so, after they are there you can use Flight Finder to book the flights. You might have to call Frontier Customer Service if it is handled any other way.