if your rear bumpers are dirty where the sensors are, or covered with ice or snow, you will get this until the bumper is cleaned or cleared from ice/snow/mud etc. The SBZA is part of the LCA system. Here's what it says in my owner's manual, you may want to check your manual to see if it is the same:
LCA may not operate when the LCA sensors
in the left or right corners of the rear
bumper are covered with mud, dirt, snow,
ice, or slush, or in heavy rainstorms. For
cleaning instructions, see "Washing the
Vehicle" under Exterior Care 0 317. If the DIC
still displays the system unavailable
message after cleaning both sides of the
vehicle toward the rear corners of the
vehicle, see your dealer.