In Dine My Way, choose "View or Book Individual Daily Reservations," then choose the green "+Add Reservation." On the "You might be double-booking" screen, select "Okay." On the next page, select "Specialty Dining." You should be able to make a reservation from this point.
For some reason, choosing "Explore Dining Options" on the first page always seems to result in the "Please check back closer to your journey date to use this feature" no matter how close you are to sailing. Hope this is helpful🙂