def METHOD_NAME(self) -> Optional[str]:
    headers = {"Content-Type": "application/x-www-form-urlencoded"}
    url = f"https://{self._domain}/api/{API_VERSION}/login"
    try:
        resp = requests.post(url=url, headers=headers, data=f"client_id={self._client_id}&client_secret={self._client_secret}")
        if resp.status_code != 200:
            return "Unable to connect to the Looker API. Please check your credentials."
    except ConnectionError as error:
        return str(error)
    data = resp.json()
    self._access_token = data["access_token"]
    self._token_expiry_date = pendulum.now().add(seconds=data["expires_in"])
    return None