Tkinter (dynamic) Frame doesn't show up
In fact, your frame is successfully added but is zero size (and moreover invisible!).
You might either add some content in, or ask the layout manager to give it all the available space MainFrame.pack(expand=True, fill="both") .
To make it visible, borderwith (alias bd) is not sufficient. It must be combined with relief (for instance relief=SUNKEN). Another way to "see" your frame is to set a background.