mega 2560 r3 only works without one of 22p caps of crystal of atmega16u2

I can imagine that the capacitor was not properly soldered to the board or is somehow short-circuited. They aren't very critical, so if the board works without the capacitor, you can probably go ahead and use it. The board may be unreliable in operation (e.g. it could just stop working) but there is very little chance you would damage anything by using it.

Can you return the board under warranty?