Radio buttons can't be clicked on tkinter/python app

I have the following code which sets up some button and radio buttons. The radio buttons (Full Tcl version info: 8.6.11) can not be clicked on my side (Python 3.9.2 on Debian Linux 11):

import tkinter as tkfrom tkinter import ttkclass ComplexGUIExample(tk.Tk):    def __init__(self):        super().__init__()        self.title("Complex GUI Example")        self.geometry("400x300")        # Frame for VPN controls        vpn_frame = tk.Frame(self)        vpn_frame.pack(pady=(10, 0), padx=(10, 0), fill=tk.X)        # VPN Connection Button        button_connect_vpn = tk.Button(vpn_frame, text="Connect VPN", command=self.dummy_command)        button_connect_vpn.pack(side=tk.LEFT, padx=(10, 10))        # Check VPN Status Button        button_check_vpn = tk.Button(vpn_frame, text="Check VPN Status", command=self.dummy_command)        button_check_vpn.pack(side=tk.LEFT, padx=(10, 10))        # Frame for URL Loading Preferences (Radio Buttons)        url_pref_frame = tk.LabelFrame(self, text="URL Loading Preferences", padx=10, pady=10)        url_pref_frame.pack(pady=(20, 0), padx=(10, 0), fill=tk.X)        # Variable to track the radio button selection        self.url_loading_preference = tk.StringVar(value="Most Recent")        # Radio Buttons for URL Loading Preference        tk.Radiobutton(url_pref_frame, text="Most Recent", variable=self.url_loading_preference, value="Most Recent", command=self.on_radio_change).pack(anchor=tk.W)        tk.Radiobutton(url_pref_frame, text="Oldest", variable=self.url_loading_preference, value="Oldest", command=self.on_radio_change).pack(anchor=tk.W)        tk.Radiobutton(url_pref_frame, text="Random page", variable=self.url_loading_preference, value="Random page", command=self.on_radio_change).pack(anchor=tk.W)    def dummy_command(self):        print("Button clicked")    def on_radio_change(self):        print(f"Selected URL Loading Preference: {self.url_loading_preference.get()}")if __name__ == "__main__":    app = ComplexGUIExample()    app.mainloop()

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How do I make the radio buttons clickable ?

I have tried using some other widgets but so far I haven't been able to make it work.