ValueError: substring not found

[hoangthai10788]

Hello,

I encountered this issue when self-learning Python. Could anyone give me a full explanation what is happening here? Any help would be greatly appreciated. Thank you!

def shift(char, key):
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    updated_alphabet = ""
    if int(key) > 0:
        for i in range(len(alphabet)):
            updated_alphabet += alphabet[(int(key) + i) % 26]
    char_index = alphabet.index(char.lower())
    new_char = updated_alphabet[char_index]
    return new_char
def main():
    message = "Please help me with this."
    new_mess = ""
    for char in message:
        new_mess += shift(char, 1)
    return new_mess
main()