Certain listings on Hipcamp will show up as "not instantly bookable on Hipcamp" (example below).
This is because they are public campgrounds owned by the federal government, which means we don't yet have access to allow booking directly from Hipcamp. We hope to down the road as government data about our parks is becoming increasingly more open and accessible.
When you click "Why?" (shown above) on the listing you're viewing, information about that park will show up, including contact information for the park or park ranger, which is where you can book and find out more about availability.