Can you remove the X-HTTP-Method key and MERGE value from the Headers? This suggests that you want to update an existing item. If I understand correctly you want to create a new list item?

Below is an example of the REST API documentation, if that helps?

https://learn.microsoft.com/en-us/sharepoint/dev/sp-add-ins/working-with-lists-and-list-items-with-rest#create-list-item

In case of an update you would also want to refer to the id of the item in your URI
POST https://{site_url}/_api/web/lists/GetByTitle('Test')/items({item_id})

https://learn.microsoft.com/en-us/sharepoint/dev/sp-add-ins/working-with-lists-and-list-items-with-rest#update-list-item