A number of issues can cause this:
1) Your reservation time is in the past or within 4 hours of the current local time where the telescope is located. All reservations must be made at least 4 hours from the current local telescope time.
2) The reservation system for this telescope may currently be offline. You can check this by viewing the status for the telescope on the Launchpad. If the status next to the Telescope you are using currently says "Offline", "Closed: System Maintenance", or "System Busy" we are likely performing maintenance and have reservations disabled on that system.
3) Another member has already filled the spot. Our reservation calendar displays the telescope's current reservations based on it's status at the time you log in to the system. If another member is already logged in or logs in while you are creating your reservation, but before you have "saved" it, they may create a reservation while you are working on creating yours. As the reservation works on a first come, first served, basis, the system accepts the first time received. You can tell if this is the case by clicking refresh on your browser, as this will update the calendar.
4) You are trying to create a reservation during observatory closed times. The reservation calendar will deny any reservation that will either begin before the sun has reached -10º in the evening - when the roof opens, or ends after the sun has reached -10º in the morning - when the roof closes. This is marked on the reservation calendar for only the current day by default, but if you check the box next to "Show Almanac Columns" it will show this period for all days you are currently viewing.
5) You have made a change to your account information (payment method, address, password, account renewed, account cancelled, plan change, etc) within the last 30 minutes. We must resync your account to all telescopes after any change to the account.
6) If you have checked all of these items and are still unable to create a reservation, please contact support for assistance as your account may need to be resynced with the telescope's computer.
7) If you have created a reservation, but need to edit it, please check this FAQ here.