It seems like you may be in need of a fixed position so that the div scrolls down with you as you navigate the page. However, it appears you may need to provide more details about the specific div you are attempting to center. Is it accurate to assume that the div follows as you scroll but is not aligned properly?
If that is the case, my assumption is that the issue lies in the margins not accounting for the height and width of the div you are trying to center in the middle of the screen. For example, if you have a div with a width of 100px, you should subtract half of that from the margin-left property to ensure the div is correctly positioned at the center of the screen. While additional context would be beneficial, I hope this explanation offers some guidance for your task.
For illustration purposes, your div could be styled like this:
.center-bottom {
position:fixed;
bottom:0px;
left:50%;
width: 100px;
margin-left: -50px;
}