What you tried, App.switch_frame(ScrollFrame) , won't work because it is calling the function from the class not the instance, which is not the window you are seeing. So you need to call the function from the instance 'app'.

This is the general problem with OOP. You often need the classes to interact in some way which is only possible if each had a reference to the other or the instances were global. Both solutions defeat the purpose/advantage of using OOP.

For your case, you could give the class FunctionsSidebarFrame a reference to the app instance which you can then use to call its swith_frame function.

class FunctionsSidebarFrame(customtkinter.CTkFrame):
    def __init__(self, master, app):
        self.app = app
        ...
    def viewGames(self):
        self.app.switch_frame(ScrollFrame)

This will also require you to give the class SidebarFrame the reference to be passed to FunctionsSidebarFrame.