My 60 day window was today, and I was lucky to get what I wanted, even heavy hitters. But it only worked for me on my app on my phone. The website was dead for me. One hint is to try selecting a time as opposed to the "breakfast" " lunch" or "dinner" buttons.
You don't have to be "checked in" to your room reservation, but it does help to be signed in to your account. My reservation windows opened at 6AM.
OP, were you able to book?