Magento Create an empty cart for login customer by REST API, You need to first fetch the current customer active quote id by V1/carts/mine endpoint.
By using given endpoint, You will get the current customer quote id with a simple API and if the customer doesn’t have any active cart, it will create an empty cart with the use of API.
Header: Authorization: Bearer <CUSTOMER_TOKEN>
You can check the link to Generate Customer Token by Rest API.
Response: Quote ID. (Example: 12 Here 12 is the quote id for the customer.)
The response of the API will be helpful to add items to the cart for the login customer with Rest API.