Deleting HD APPLE SSD SM0128GB Media

I have recently bought a new MacBook Air and are thus planning on selling my old MacBook Pro 13" Early 2015. When trying to delete the disc called "APPLE SSD SM 0128GB Media" through the Disk Utility Tool I get a message saying that it cannot "open unit. : (69877)" and that the action has failed. What should I do in order to delete the HD?