Every school has its own policies, but many will not let you put off a prerequisite until the summer before you attend. Many of them have a preterm that starts in July, and that conflicts with possible summer courses. It also radically raises the chances that you will fail that class because of senioritis and they will be forced to rescind your admission and find a last-minute replacement.
If you are hoping to get into the class of 2023 or later, then you can relax because you are totally allowed to have unfinished prereqs at the time of application. When you fill out the app it will give you a place to put planned future courses, and they will be changeable during the academic update period starting the November after you apply.