Maybe you forgot the high brake light? that is part of the tail/parking lights too?

The others are right a corroded bulb/socket can add resistance and mess up the works. Clean the bulb and socket with dry scotch brite and use a little di-electric silicone to prevent corrosion.

If it comes on with parking lights, look there first.

Sometimes you may find that bulbs need to be replaced in pairs, since one may
draw more current than another triggering the bulb sensor.

Try removing the tail lights a set at a time, until the bulb out sensor is happy.
Then you find the problem. The sensor can trigger with both bulbs lit, but one
bulb will be drawing more current.

Bill