import tkinter as tk
from tkinter import Tk


root = tk.Tk()
root.title("Converter")
root.geometry('300x300')
l = tk.Label(root, text='Сколько километров нужно перевести?', pady=3, width=300, height=2, bg="blue",fg="black",font=25)
l.pack()

km = tk.StringVar()
mile = tk.StringVar()

e = tk.Entry(root, textvariable=km, justify='center', width=150, bg="cyan2")
e.pack(fill='x')

def converter():
    try:
        mile.set(float(km.get()) * 0.621371)
    except:
        mile.set('Не получилось конвертировать')


l2 = tk.Label(root, text="Мили", bg="blue", fg="black", font=25)
l2.pack(fill='x')
e2 = tk.Entry(root, textvariable=mile, justify='center', width=150, bg="cyan2")
e2.pack(fill='x')

b = tk.Button(root, text='Перевести', command=converter)
b.pack(fill='x')

root.mainloop()