how to solve "SyntaxError: Unexpected token < in JSON at position 0" while using posting data while fetch?

I was using fetch post method to post this array of object to my backend. But dont know why I am getting this error.

const inventory = [
    {
        name: "phone",
        price: 1200,
    },
    {
        name: "Laptop",
        price: 12000,
    },
    {
        name: "I mac",
        price: 120000,
    },
]

fetch('localhost:5000/inventory', {
    method: 'POST',
    header: {
        'content-type': 'application/json'
    },
    body: JSON.stringify(inventory)
})