To see a greeting when you click on the button then this is the correct code

Tkinter and datetime are in-built-module in Python. You don’t need to install it.

import datetime
import tkinter as tk
from tkinter import messagebox

now = datetime.datetime.now()
current_hour = now.hour

def classify_time_of_day():
    if 6 <= current_hour < 12:
        greeting = 'Guten Morgen'
    elif 12 <= current_hour < 18:
        greeting = 'Guten Tag'
    elif 18 <= current_hour < 20:
        greeting = 'Guten Abend'
    else:
        greeting = 'Gute Nacht'
    return greeting

root = tk.Tk()
root.geometry("700x700")
root.title("Tagesbegrüßung")  # Titel des Fensters hinzufügen

# Label für die Anzeige des Ergebnisses
result_label = tk.Label(root, text="", font=("Arial", 16)) # Schriftgröße angepasst
result_label.pack(pady=20) # Padding für bessere Optik

def show_greeting():
    greeting = classify_time_of_day()
    result_label.config(text=greeting)  # Text des Labels aktualisieren

# Button zum Auslösen der Begrüßung
show_button = tk.Button(root, text="Zeige Begrüßung", command=show_greeting)
show_button.pack()

root.mainloop()