Getting Unexpected token r in JSON at position 0 error.

Here is my code :

import React, { useEffect, useState } from 'react';

const AddInventory = () => {

    const [items, setItems] = useState([]);
    const [isReload, setIsReload] = useState(false);

    useEffect(() => {
        fetch("http://localhost:5000/inventory")
            .then((res) => res.json())
            .then((data) => setItems(data));
    }, [isReload]);

    const handler = (e) => {
        e.preventDefault();
        const name = e.target.name.value;
        const email = e.target.email.value;
        const user = { name, email };

        fetch("http://localhost:5000/inventory", {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json',
            },
            body: JSON.stringify(user),
        })
            .then(response => response.json())
            .then(data => {
                const newUsers = [...items, data]
                setItems(newUsers)
                setIsReload(!isReload)
            })
            .catch((error) => {
                console.error('Error:', error);
            });
    }

    return (
        <div className='container mt-5'>
            <h4>this is add inventory {items.length}</h4>
            <form onSubmit={handler} action="">
                <input className='m-2' type="text" name="name" id="" placeholder='name' />
                <input className='m-2' type="text" name="email" id="" placeholder='email' />
                <input className='m-2' type="submit" value="Add User" />
            </form>
            {
                items.map(item => <li key={item._id}>Name:{item.name}    Email:{item.email}</li>)
            }
        </div>
    );
};

export default AddInventory;

but i'm trying to understand what's wrong with my code. Maybe there is another probably that the server-side functionality that is usually responsible for generating and returning the stringified JSON failed.