* Second - If not the first answer. Check your z-index. Changing the z-index will adjust the layering of your screen with CSS, and may need to be changed depending on how your theme displays the backgrounds and colors. For more about z-index - https://www.w3schools.com/cssref/pr_p...
you should use a container element and add a background image to this container. Then you should use the “background-attachment: fixed”CSS property to create the actual parallax effect. You can find a working