Headphone jack

In the jack there's a microswitch that senses the presence of the plug: probably got stuck cause of dust, lint or other contaminant. Flash a light and verify there's nothing in there. If there is, you'll probably need to take it to service for help. If there isn't try blowing a few "shots" of compressed air from one of those 'duster' cans and/or work a headphone plug in and out several times to try and unstick the switch.

The can of compressed air? A few bucks.

The service visit? Depends.

Evaluation is free, they will determine what's wrong.

Best case, it is something easily extracted or a dirty switch, minimal service fee.

Worst case, the jack is permanently damaged and the phone needs replacement. A 6s will likely have no warranty left, probably better off getting something newer.