If you are not able to see the Parallax option in the Beaver Builder, chances are you』re using the free / lite version of Beaver Builder which does not offer the parallax feature. If you need the parallax effect along with other features of Beaver Builder, you may consider purchasing it from here.
However, should you wish to stick with the lite version, you may keep the background image fixed as displayed in the screenshot below. It will create an effect that will be very similar to parallax.