PayPal API Billing Authentication Failure (Python Flask)

I'm encountering an authentication failure issue while using the PayPal API to create billing plans and handle subscriptions. Here's the error message I'm receiving:

{"name":"AUTHENTICATION_FAILURE","message":"Authentication failed due to invalid authentication credentials or a missing Authorization header.","links":[{"href":"https://developer.paypal.com/docs/api/overview/#error","rel":"information_link"}]}

This error occurs when attempting to access the URLhttps://api.sandbox.paypal.com/v1/billing/plans/P-054793526X109145CMXEAUWQ

Below is the my code:

def get_access_token(client_id, client_secret):    auth_url = 'https://api.sandbox.paypal.com/v1/oauth2/token'    headers = {'Accept': 'application/json','Accept-Language': 'en_US',    }    data = {'grant_type': 'client_credentials'    }    auth = (client_id, client_secret)    response = requests.post(auth_url, headers=headers, data=data, auth=auth)    if response.status_code == 200:        access_token_data = response.json()        access_token = access_token_data.get('access_token')        if access_token:            return access_token        else:            print("Failed to obtain access token: Access token not found in response.")            return None    else:        print(f"Failed to obtain access token: {response.text}")        return None@app.route('/purchase', methods=['POST'])@login_requireddef purchase_subscription():    plan = request.form.get('plan')    selected_gateway = request.form.get('payment_gateway')    plan_settings = get_plan_settings()    if not plan_settings:        flash('Failed to fetch plan settings from the database.', 'danger')        return redirect(url_for('subscription'))    credits_to_add = 0    credits_price = 0    subscription_type = None    subscription_start_date = None    subscription_end_date = None    subscription_type_mapping = {'plan1': 'monthly','plan2': 'monthly','plan3': 'monthly','plan4': 'yearly','plan5': 'yearly','plan6': 'yearly'    }    if plan in subscription_type_mapping:        subscription_type = subscription_type_mapping[plan]        if subscription_type == 'monthly':            frequency = "MONTH"        elif subscription_type == 'yearly':            frequency = "YEAR"        credits_to_add = plan_settings[plan +"_credits"]        credits_price = plan_settings[plan +"_price"]    else:        flash('Invalid plan selected.', 'danger')        return redirect(url_for('subscription'))    ACCESS_TOKEN = get_access_token(PAYPAL_CLIENT_ID, PAYPAL_CLIENT_SECRET)    if ACCESS_TOKEN:        print("Access token obtained successfully:", ACCESS_TOKEN)        product_id = create_product(plan, credits_price, ACCESS_TOKEN)        if product_id:            billing_plan = create_billing_plan(plan, frequency, credits_price, product_id, ACCESS_TOKEN)            if billing_plan:                print("Redirecting to: ", billing_plan)                return redirect(billing_plan)            else:                flash('Failed to initiate payment.', 'danger')                return redirect(url_for('subscription'))        else:            flash('Failed to create product.', 'danger')            return redirect(url_for('subscription'))    else:        print("Failed to obtain access token.")        flash('Failed to obtain access token.', 'danger')        return redirect(url_for('subscription'))def create_product(plan, amount, access_token):    auth_url = 'https://api.sandbox.paypal.com/v1/oauth2/token'    product_url = 'https://api.sandbox.paypal.com/v1/catalogs/products'    print(f"Access token in app.py {access_token}")    headers = {'Content-Type': 'application/json','Accept-Language': 'en_US','Authorization': f'Bearer {access_token}'    }    # Create product    product_data = {"name": f"{plan.capitalize()} Subscription","description": f"{plan.capitalize()} Subscription","type": "SERVICE","category": "SOFTWARE"    }    response = requests.post(product_url, headers=headers, json=product_data)    if response.status_code == 201:        return response.json().get('id')    else:        print(f"Failed to create product: {response.text}")        return Nonedef create_billing_plan(plan, frequency, amount, product_id, access_token):    auth_url = 'https://api.sandbox.paypal.com/v1/oauth2/token'    plan_url = 'https://api.sandbox.paypal.com/v1/billing/plans'    headers = {'Accept': 'application/json','Accept-Language': 'en_US','Authorization': f'Bearer {access_token}'    }    # Create billing plan    plan_data = {"product_id": product_id,"name": f"{plan.capitalize()} Subscription","description": f"{plan.capitalize()} Subscription","billing_cycles": [{"frequency": {"interval_unit": frequency,"interval_count": 1            },"tenure_type": "REGULAR","sequence": 1,"total_cycles": 0,"pricing_scheme": {"fixed_price": {"value": str(amount),"currency_code": "USD"                }            }        }],"payment_preferences": {"auto_bill_outstanding": True,"setup_fee": {"value": "0","currency_code": "USD"            },"setup_fee_failure_action": "CONTINUE","payment_failure_threshold": 3        },"quantity_supported": False,"taxes": {"percentage": "0","inclusive": False        }    }    print("Plan data (params): ",plan_data)    print("Plan url: ",plan_url)    response = requests.post(plan_url, headers=headers, json=plan_data)    if response.status_code == 201:        print("Response: ", response)        plan_response = response.json()        for link in plan_response.get('links', []):            if link.get('rel') == 'self':                return link.get('href')        # If no URL with rel='self' is found        print("Failed to find billing plan URL in response.")        return None    else:        print(f"Failed to create billing plan: {response.text}")        return None

Console log:

Access token obtained successfully: HIDDENAccess token in app.py: HIDDENPlan data (params):  {'product_id': 'PROD-6T149159N1656222J', 'name': 'Plan2 Subscription', 'description': 'Plan2 Subscription', 'billing_cycles': [{'frequency': {'interval_unit': 'MONTH', 'interval_count': 1}, 'tenure_type': 'REGULAR', 'sequence': 1, 'total_cycles': 0, 'pricing_scheme': {'fixed_price': {'value': '19.99', 'currency_code': 'USD'}}}],'payment_preferences': {'auto_bill_outstanding': True, 'setup_fee': {'value': '0', 'currency_code': 'USD'}, 'setup_fee_failure_action': 'CONTINUE', 'payment_failure_threshold': 3}, 'quantity_supported': False, 'taxes': {'percentage': '0', 'inclusive': False}}Plan url:  https://api.sandbox.paypal.com/v1/billing/plansResponse:  <Response [201]>Redirecting to:  https://api.sandbox.paypal.com/v1/billing/plans/P-7LY37792X0654533CMXESLWQ

Im' 100% sure the access token & client id and secret are correctAny insights or suggestions on how to troubleshoot and resolve this issue would be greatly appreciated.

Thank you!