Have you checked the upper lights above the trunk?  If ok, unfortunately, a
lot of times it will be a corroded connection on the light bulb bases, and
you have to clean the base and sometimes the socket.  Other times, the bulb
will still light, but the resistance inside has changed enough to cause the
dash light to come on. Look for bulbs that are real black inside.

In almost every case where the rear light bulb failure indicator is
illuminated, one of the rear bulbs has failed.

As Sam in Savannah suggested, check all of the bulbs in the center high
mounted stop lamp in the rear window.

Also check:
Both rear license plate bulbs
Both left and right stop lamps
Both left and right tail lights
Both left and right turn signals
Both left and right back up lights.

In some cases, there are two bulbs in a fixture so check carefully.

If all of the bulbs are actually good and illuminated, check the bulb
fixture grounds and check the sockets for corrosion.