Elements ignore the Position: Fixed for align in CSS

i have a problem in HTML and CSS, i have a lateral left menu with a 100% height and position relative. I want to put other elements at the side of it, it is not the problem, the problem is that if I want, for example, center the other element to the center between the menu and the right, I can't do it. It does a center between left and right of the document.

Elements that are in the left of the page are not shown because are behind the menu. How can I indicate that the menu "exists"?