2016 Mustang V6 will crank but not start

This is almost certainly a failure in the fuel pump or fuel pressure regulator.

Both are mounted in the fuel tank, what I don't know, as I haven't taken the tank down on an S550 (or opened the access panel if it has one) is whether they're separate or sold as a unit.

I see and/or fix similar issues somewhat frequently on the BMWs I work on for a living, and have seen it on other makes as well when working at other dealerships.

There are other possibilities, but all a tech would need to do to either diagnose it or get started down the right path is let it sit overnight and then press the start button without pressing the brake while the scan tool is hooked up. If it isn't still primed from the previous start, the system is bleeding down excessively. If it doesn't build the correct amount of pressure on the first try, and takes a few to do so, either the fuel pump is weak, the fuel strainer is clogged, a hose or tube inside the tank has split, or the fuel pressure regulator has failed.

Ford more than likely has a test that will let said technician figure it out if he knows what he's doing.

Being an Ecoboost, there is also the possiblity of the high-pressure fuel pump mounted to the valve cover having failed, but that will usually turn on the check engine light if there's a sensor attached to it (the HPFP failing can cause the air/fuel ratio to lean out and damage the engine, so most manufacturers monitor it directly).