There are two reasons why an item could be marked as sold.
You have Track Quantity enabled on that item and the Quantity is set to 0. To edit a registration item to update those values navigation to Registration and then Items.
Your total golfer/participant cap has been met. In this case, navigate to Registration and then Golfer Settings and adjust the Total Golfers.
After adjusting the settings that was causing the issue, registrants will now be able to register for that item.