I'm curious as to what the actual issue is:
1. Is it that the system keeps kicking the OP out, etc.?
2. Is it that the OP is trying to reserve past the 6 month open res period (yes, Nevada State Beach has the 6 month limit)?
3. Is it that the OP is trying to reserve before the campground open date of May 12, 2017?
4. Is it that the OP is trying for specific dates and the system is indicating that there are no available campsites for those dates?
If #1, the OP should try contacting the campground directly.
If #2 or #3, the OP needs to pick appropriate dates between May 12, 2017 and 6 months from the date he tries to make the reservation or wait until his desired dates fall within the 6 month period AND/OR ensure the campground is open for the dates he wants.
If #4, the OP should review the list by "date range availability" and see what the reservations look like during the dates he wants. It may be that there is no one campsite available for all the dates, but that he could move after the first X days to another site and get all the dates reserved.