Input error correction.

[menator01]

Please post code using the bbtags. Makes it easier to help.

Is this what you are trying for?
Added a try block

import tkinter as tk
from tkinter import ttk
from tkinter.messagebox import showinfo, showerror
from datetime import datetime

# root window
root = tk.Tk()
root.geometry("300x200")
root.resizable(False, False)
root.title('Time Clock')

# store StartofWork address and EndofWork
StartofWork = tk.StringVar()
EndofWork = tk.StringVar()


def login_clicked():
    """ callback when the login button clicked
    """
    WorkStart = StartofWork.get()
    tdelta = timediff()

    if tdelta:
        """
        msg = f'Start of Shift: {StartofWork.get()} and End of Shift: {EndofWork.get()}'
        """
        msg = f'Total Hours: {tdelta}'
        showinfo(
        title='Time Calculated',
        message=msg
        )


def timediff():
    WorkStart = StartofWork.get()
    """StartTime = input("Enter Start Time in HH:MM\n") """
    print("This is WorkStart", WorkStart)
    print("This is EndofWork", EndofWork.get())
    FMT = '%H:%M'
    try:
        tdelta = datetime.strptime(EndofWork.get(), FMT) - datetime.strptime(StartofWork.get(), FMT)
        return tdelta
    except ValueError:
        showerror('Error!', 'Time format is not correct.')

# Sign in frame
signin = ttk.Frame(root)
signin.pack(padx=10, pady=10, fill='x', expand=True)


# StartofWork
StartofWork_label = ttk.Label(signin, text="Clock In:")
StartofWork_label.pack(fill='x', expand=True)

StartofWork_entry = ttk.Entry(signin, textvariable=StartofWork)
StartofWork_entry.pack(fill='x', expand=True)
StartofWork_entry.focus()

# EndofWork
password_label = ttk.Label(signin, text="Clock Out:")
password_label.pack(fill='x', expand=True)

password_entry = ttk.Entry(signin, textvariable=EndofWork)
password_entry.pack(fill='x', expand=True)

# login button
login_button = ttk.Button(signin, text="Calculate Time", command=login_clicked)
login_button.pack(fill='x', expand=True, pady=10)

root.mainloop()