Humm... the first weird thing I'm seeing is the driver icon in the middle of loco (I ever saw it in the front). The only thing that comes to my mind is: before add the driver icon be sure the engine is not checked as "broken" (click twice the engine to open its flyout).
Edited later: forget my suggestion because if the loco is broken you cannot add a driver, and if the driver is already added you cannot break the loco.
In fact the checkable box icon (Player Consist is its name) is used to assign a "Player Train" in standart scenarios. If you are in a freeroam scenario that icon is always grayed 'cuz any train with driver icon is drivable.

But if you are in a standart scenario, then

Who doesn't have dog, hunts with cat.