Slide the tray back into the slot. It should slide in smoothly. Once it is almost flush, you will feel resistance from the gasket and the spring lock. Push firmly until you hear a distinct click and the tray sits flush with the phone’s body. If the tray is not flush, your waterproofing is compromised.
Do not yank the tray out. Pull it out gently with your fingernails. If it feels stuck, wiggle it slightly left to right, but never force it.