'str' object has no attribute 'decode' when decrypt aes

so i have function to decrypt aes ctr. but when i tried to decrypt, the result i got from plaintext is type bytes. i want to get the result of plaintext using decode, because when i encrypt it, i was encode it first.

here's my encrypt func:

def encrypt(key, pt):
    plaintext = read_file(pt)
    if isinstance(plaintext, str):
        pt= plaintext.encode("utf-8")
        print(pt)
        print(type(pt))

    if len(key) <= key_bytes:
        for x in range(len(key),key_bytes):
            key = key + "0"

    assert len(key) == key_bytes
    
    # Choose a random, 16-byte IV.
    iv = Random.new().read(AES.block_size)

    # Convert the IV to a Python integer.
    iv_int = int(binascii.hexlify(iv), 16)
    print(iv_int)

    # Create a new Counter object with IV = iv_int.
    ctr = Counter.new(AES.block_size * 8, initial_value=iv_int)

    # Create AES-CTR cipher.
    aes = AES.new(key.encode('utf8'), AES.MODE_CTR, counter=ctr)

    # Encrypt and return IV and ciphertext.
    ciphertext = aes.encrypt(pt)
    return (iv, ciphertext)

and my decrypt func :

def decrypt(key, iv, ciphertext):
    assert len(key) == key_bytes
    print(key)
    print(iv)
    print(ciphertext)                                                                       
    print(type(ciphertext))
    # Initialize counter for decryption. iv should be the same as the output of
    # encrypt().
    iv_int = int(binascii.hexlify(iv), 16)
    print(iv_int)
    ctr = Counter.new(AES.block_size * 8, initial_value=iv_int)

    # Create AES-CTR cipher.
    aes = AES.new(key.encode('utf8'), AES.MODE_CTR, counter=ctr)

    # Decrypt and return the plaintext.
    plaintext = aes.decrypt(ciphertext)
    if isinstance(plaintext, bytes):
        pt= plaintext.decode("ascii")
        print(pt)
        print(type(pt))
    return pt

the result from plaintext : b'\xf6\x18n:\x8a\xb6\x8e\xb1\xb1'

what i want to get from plaintext / pt : SandI2021

and my traceback:

Traceback (most recent call last): File

"C:\Users\Capoo\lib\site-packages\django\core\handlers\exception.py", line 55, in inner response = get_response(request) File "C:\Users\Capoo\lib\site-packages\django\core\handlers\base.py", line 197, in _get_response response = wrapped_callback(request, *callback_args, **callback_kwargs) File "F:\KULIAH\SEMESTER8\SKRIPSI\MusicLockApp\MusicLockApp\views.py", line 171, in decode final = password.decode() AttributeError: 'str' object has no attribute 'decode'