Hi and welcome,
The high level brake light is not bulb based, but is an LED strip. So usually only one element fails and the others carry on as normal. So it seems likely that the supply to the strip is the problem. As a result it looks like the usual suspect to check, connections, fuse, stop switch.
As an aside, does anyone know when an LED strip stop being illegal when elements start to fail, one gone, two, most ??
B-Max Zetec 1.4 Petrol
Mid 2015 build
Pretend Spare Wheel