试试这个：

from tkinter import *

def create_main_window():
    global main_window
    main_window = Toplevel(main_window)
    label = Label(main_window, text=f"{place_entry.get()}")
    label.pack()
    # main_window.update() # This is useless

entrance_window = Tk()

first_text_label = Label(entrance_window, text="You are in:")
first_text_label.grid(row=0, column=0)

place_entry = Entry(entrance_window)
place_entry.grid(row=0, column=1)

submit_button = Button(entrance_window, text="Submit", command=create_main_window)
submit_button.grid(row=1, column=0, columnspan=2)
entrance_window.mainloop()

我将标签创建移到了create_main_window中。另外请注意，使用var = a().b()时，会将b()返回的内容保存到var中。这就是为什么当您使用var = Entry(...).pack(...)时，var始终为None。

这是因为你试图给一个不存在的对象添加一个Label，把Label函数移到create_main_window（）函数中，如下所示：

from tkinter import *

def create_main_window():
    global main_window, entrance_window
    main_window = Toplevel()
    place_entry = Entry(entrance_window).grid(row=0, column=1)
    Label(main_window, text=f"{place_entry}").pack()
    main_window.update()

entrance_window = Tk()
first_text_label = Label(entrance_window, text="you are in:").grid(row=0, column=0)

submit_button = Button(entrance_window, text="Submit", command=create_main_window).grid(row=1, column=0, columnspan=2)
entrance_window.mainloop()

从未使用过global。不要使用通配符from tkinter import *
要查看Label小工具，必须将其放置在create_main_window() function内。
摘录：

import tkinter as tk

def create_main_window() -> None :   
    main_window = tk.Toplevel()
    tk.Label(main_window, text=f"{place_entry.get()}").pack()
     

entrance_window = tk.Tk()

first_text_label = tk.Label(entrance_window, text="you are in:").grid(row=0, column=0)

place_entry = tk.Entry(entrance_window)
place_entry.grid(row=0, column=1)

tk.Button(entrance_window, text="Submit", command=create_main_window).grid(row=1, column=0, columnspan=2)
 
entrance_window.mainloop()

if __name__=='__main__':
    create_main_window()

截图：