Looks nice, but do you know you can use list comprehension to shorten the code? I have made an example, also the fancy way in which I have defined the function is called type hinting incase if you don't know:

def main():
'''
The main function to be executed in the program.
'''

choice = input("Type 'encode' to encrypt, Type 'decode' to decrypt\n").lower()
text = input("Type your message (only text message without spaces): \n").lower()
shift = int(input("Type your shift number: \n"))
if choice == "encode":
text_change(text, shift, 1)
elif choice == "decode":
text_change(text, shift, -1)
else :
print("You should have entered correct choice!!")
return 0
alphabet = ["a","b","c","d","e","f","g","h","k","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

def text_change(string: str, shift: int, mode: int) -> None:
'''
Takes in the string and shift and convert it based on the mode.

parameters:

• string (str type): The text to encode or decode
• shift (int type): The shift to apply on the message, its like a special key
• mode (int type): 1 for encode, -1 for decode
• returns nothing (None)
  '''
  print([chr(ord(i) + mode shift) for i in list(string)], sep = "", end = "")
  if name == "main":
  main()