One black spot and line showing at the top of 55 inches curved UHD Samsung TV. Is anyone tell the reason?

The TV may not be overscanning as much as it should. If the TV doesn't have a user menu to adjust for this then you should be able to get it done by a tech under warranty. If it's out of warranty then try searching for the service manual online. You usually need to entire a sequence of button pushes to enter that menu.