This is the single most common mistake made on the Outer Cape. Drivers follow signs to Coast Guard Beach in July, reach the end of Doane Road, and find the beach turnaround closed to general parking.
Through the high summer season you park at the inland lot off Doane Road and take the shuttle bus down to the sand. It runs on a loop, it is included once you have paid the National Seashore vehicle fee, and it has a last run in the early evening that is worth knowing before you settle in for the sunset. Dates, hours and frequency are set each season, so confirm at Salt Pond Visitor Center rather than trusting a figure you read in March.
The alternative is the one locals use. The Nauset Trail runs about a mile and a half of paved path from Salt Pond Visitor Center straight to the beach, mostly flat, through pitch pine and past the edge of the marsh. On a bike you arrive at the racks at the top of the ramp, you never queue, and you pay the cyclist rate rather than the vehicle rate.
Outside the shuttle season the beach turnaround itself opens for parking, and there is room for perhaps a few dozen cars. In October that is plenty. In August it would be gone by half past seven, which is precisely why it is closed.