The most common reason for the tire pressure warning light to stay on even with proper tire pressure is a faulty tire pressure monitoring system.
I had an issue with another vehicle where the tire pressure warning light would stay on and not go off even when the tire pressure was correct.
I took it to a shop and they replaced the tire pressure monitoring system and it fixed it.
If the light is constant then inflating to the correct placard pressure should turn it off.
If this is not the case then this indicates a puncture.
If the light is intermittent or if it stays on after correct inflation or replacement of a punctured tire then this indicates a fault with the TPMS system.
You also may need to have the tire pressure warning light reset by a shop.
Unfortunately, Subaru doesn't install a TPMS reset switch on cars sold in the U.S.
If the warning light remains illuminated even after you've confirmed that your tires are inflated to the correct pressure, you'll need to visit a tire shop or a dealership to have the system reset.
You can also reset the tire pressure warning light yourself.
Hold the TPMS reset button until the tire pressure light blinks three times, then release it.
Start the car and let it run for 20 minutes, which allows the sensors to reset.
You'll find the tire pressure monitor reset button located beneath the steering wheel.