When your character is equipped with a health kit, you can see that in your character's hand. Press and hold R1 so your character goes into an animation of healing (bandaging) until the animation stops. Do not release R1 until the animation stops.
If you are not equipped with a health kit in your hand, pressing R1 does nothing to heal.
Make sure that you're indeed equipped with health kit. You can check visually.
If there is no health kit, obviously you can't equip one. But you can craft one.