Getting Status 401 Unauthenticated while calling app store server notifications to send a test notification

I'm trying to create a test notification, but getting Unauthorized from the API. I've created an In-App Purchase key, and using the following code to generate the JWT Token:

from datetime import datetime, timedelta
from time import time, mktime
import jwt

dt = datetime.now() + timedelta(minutes=19)

headers = {
    "alg": "ES256",
    "kid": "B7J<..>", 
    "typ": "JWT",
}

payload = {
    "iss": "acaf4d66-ea<..>",
    "iat": int(time()),
    "exp": int(mktime(dt.timetuple())),
    "aud": "appstoreconnect-v1",
    "bid": "com.w<..>"
}


with open("SubscriptionKey_B7J<..>.p8", "rb") as fh: 
    signing_key = fh.read()

gen_jwt = jwt.encode(payload, signing_key, algorithm="ES256", headers=headers)

print(f"{gen_jwt}")

One of the request IDs that failed: FPJYAOSUZTFXXIKUELW6QE7F54.0.0