The function "fetch" with local testing does not work

Question

I am a newbie and I can’t understand why my request is not sent. I wrote a function, JSON is created, but when sending, an error falls and the request does not reach the server. I see that 2 requests are sent, apparently to different places and with different answers. Backand server is launched

<script>
    function submitData() {
        // Get form data
        const firstName = document.getElementById("firstName").value;
        const lastName = document.getElementById("lastName").value;


        // Construct JSON data
        const data = JSON.stringify({
            firstName: firstName,
            lastName: lastName
        });

        // Send data to server
        fetch("http://localhost:9001/client/create", {
            method: "POST",
            // mode: "no-cors",
            headers: {
                "Content-Type": "application/json"
            },
            body: data
        })
            .then(response => response.json())
            .then(data => console.log(data))
            .catch(error => console.error(error));
    }
</script>

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