The problem is that each kernel config might have prerequisites, i.e. other configs that have to be turned on before it becomes available. Turns out my problem was that the necessary prerequisites for CONFIG_SCHED_CLASS_EXT were off, so it wasn't showing up in menuconfig and couldn't be turned on using vim. To see what prerequisites you need for a command, run make menuconfig, and in the menu that is opened, type a slash "/", this opens a search bar so you can search for configurations. type out the name of your configuration, and press Enter. The opened window shows you the status of the config itself, its prerequisites, and its location in the submenus.