Steps to reproduce

1. Import [Magento] Ecommerce-Endpoint.postman_collection.json  to postman for testing. 
2. Run requests in authenticate folder for cart and checkout
3. Follow the document: https://devdocs.magento.com/redoc/2.2/#operation/quoteCartItemRepositoryV1SavePost  and  https://devdocs.magento.com/guides/v2.4/rest/tutorials/orders/order-add-items.html
4. Get customer access token
5. Put the access token into request  https://magento2demo.firebearstudio.com/rest/V1/carts/mine/items

.. Logs, screenshots, gifs...

Expected results

+ Can add an item to cart.

+ Run all requests in cart/checkout successfully.

Actual results

Workaround

Development notes

We are using free on Magento demo server:  https://magento2demo.firebearstudio.com. Maybe it's unstable and the limited permission.

https://magento.stackexchange.com/questions/244602/consumer-is-not-authorized-to-access-resources-magento-salessales

We can need a stable demo server and full of permission like Salesforce OnDemand or SAP.