DIAGNOSIS The sanitize indicator only lights up if the final rinse water reached a high enough temperature (around 155°F) for long enough. If it's not coming on, your incoming hot water might not be hot enough, or the cycle you selected doesn't include a sanitizing rinse.
FIX First, ensure you're running a cycle that includes sanitizing, like Auto, Heavy, or Sanitize. The Eco or Quick Wash cycles do not. Next, check your home's water heater. It should be set to at least 120°F. Run the hot water at the sink next to the dishwasher—it should get genuinely hot within a minute. Let the dishwasher run and check the light at the very end of the cycle (it stays on for a few hours).
IF THAT DIDN'T WORK The heating element or temperature sensor inside the dishwasher could be faulty, preventing it from boosting the water to the required temp. This is less common but would require a technician's diagnosis.
HEADS UP The sanitize light is an indicator, not a guarantee of sterilization. Its main job is to give you confidence that the wash water got hot enough to kill most common bacteria. Dishes can still be clean without the light if you're using a shorter cycle.