Cesar Cipher

[snippsat]

Some tips and as example print both with and without spaces.
Also range(len(chain_to_code)) is not good here,just remember that range(len(sequence)) is almost always a bad way.
So just iterate over the string,no adding on is needed.

ALPHABET = "abcdefghijklmnopqrstuvwxyz"
GAP = 4
chain_to_code = "ave caesar"
encoded_without_spaces = ""
encoded_with_spaces = ""

for character in chain_to_code:
    #print(repr(character)) # Tips to see all
    if character in ALPHABET:
        encoded_char = ALPHABET[(ALPHABET.index(character) + GAP) % len(ALPHABET)]
        encoded_without_spaces += encoded_char
        encoded_with_spaces += encoded_char
    # Handle spaces
    elif character == " ":
        encoded_with_spaces += " "

print(f"Encoded without spaces: {encoded_without_spaces}")
print(f"Encoded with spaces: {encoded_with_spaces}")

Output:
Encoded without spaces: ezigeiwev
Encoded with spaces: ezi geiwev

But just need this as it will handle inputs string with or without space correct.

ALPHABET = "abcdefghijklmnopqrstuvwxyz"
GAP = 4
chain_to_code = "ave caesar"
concatenated_chain = ""

concatenated_chain_with_spaces = ""
for character in chain_to_code:
    if character in ALPHABET:
        concatenated_chain += ALPHABET[(ALPHABET.index(character) + GAP) % len(ALPHABET)]
    else:
        concatenated_chain += character

print(f"Encoded: {concatenated_chain}")

Output:
Encoded: ezi geiwev