I have a problem with the API

I suspect it has to do with the format of the request. Maybe only the content type is missing. Have a look at other examples that use JavaScript for this: https://stackoverflow.com/questions/68783927/400-error-when-send-json-data-to-wordpress-admin-ajax-with-fetch-api

You can also try to write the request in PHP. Or execute it with curl. If that works, then you can narrow down the cause to writing in JavaScript.