While the API module for 15 exists there are no REST methods except one that tells the asterisk version (as an example). None of your calls will work unless you want to go code something yourself for the rest part. You are also calling them wrong as well but that doesn’t matter because it won’t work the way you want anyways.
The only parts that do work are graphql and only for adding extensions and viewing data from other modules. If said modules are supported.
The point of this module is to encourage the community to add functionality that is need or request functionality that is wanted. The reason work was not done “completely” is that there are over 128 modules. It would have taken years to get the work done.
The API module itself is a central holding place that all other modules link into. If you want user functionality for core in API under rest then you’ll need to go into the core module and add the code to process it. As it stands right now it doesn’t not exist.
It does for GraphQL however. But it is not tested there and may be harder for you to use.