Hmmm, I emailed the hotel directly and received this reply (cut and pasted):

"Greetings from Hilton Dalaman Sarıgerme Resort & Spa.

We would like to inform you that there is a systematic error that we are working on it to solve at the moment. And unfortunately we are not able to make rewards booking directly.

So if you wish, please share your spesific dates that you would like to accomodate, according to our availability we are glad to share our rates with you.

Thank you for your understanding.

If you need any further assistance please do not hesitate to contact us at any time. "

So no rewards or cash bookings on Hilton.com but contact us directly and we can make a cash booking but no rewards bookings. Maybe leaving Hilton?