How to obtain a missing part?
This part is 6047220: Plate W. Bow 2X2X2/3. You can get it from Bricklink, but first try the official LEGO replacement parts service. They will most probably send you a free replacement part and with your report you are contributing to the quality assurance project of LEGO, helping them avoid such cases in the future.

P.S. Still before, check the box, the bags, the building area and any partially completed builds for mistakes (like using it instead of a Plate W. Bow 1X2X2/3). There is a non-negligible chance your part is hiding somewhere, and everybody is much better off if you find it before you take the other steps.