In Overwatch 2, Mercy’s controls for secondary fire and ability 2 are switched on the controller options. If you use a gamepad for your left hand and a mouse in your right hand, you cannot play Mercy as you cannot boost your allies. If you have your secondary fire set on your mouse and you move on the joystick you stop boosting, if you have your ability 2 (the boost ally) for the controller set, you stop boosting if you aim. You can set both and just hold both buttons while moving and aiming, but it is the only ability that requires two buttons to be pressed to activate.