I’m building an AI bot and I want to use function calling but I don’t understand how to do it

from openai import OpenAI
import json
from datetime import datetime

client = OpenAI(api_key="key")



def get_current_datetime():
    return {"current_datetime": datetime.now().isoformat()}

functions = [
    {
        "name": "get_current_datetime",
        "description": "Возвращает текущую дату и время в формате ISO 8601.",
        "parameters": {
            "type": "object",
            "properties": {}
        }
    }
]

messages = [
    {"role": "system", "content": "Ты можешь вызывать функции для получения информации."},
    {"role": "user", "content": "Какое сейчас время?"}
]

response = client.chat.completions.create(
    model="gpt-4o",
    messages=messages,
    functions=functions,
    function_call="auto"
    
)


print(response)

To begin with, I created a simple function that returns the date and everything seems to be correct, but openai returns none