Why do I get an error when I send data to the server?

I’m creating a form to login to a site, but I’m getting an error saying the JSON is invalid.

Output upon sending:

Uncaught (in promise) SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

Code:

const handleSubmit = async (event: FormEvent<HTMLFormElement>) => {
    event.preventDefault()
    console.clear()
    const url_api = 'http://localhost:5555/api'
    const formData = new FormData(event.currentTarget)
    const res = await fetch(`${url_api}/auth/login`, {
      method: 'POST',
      headers: {'Content-Type': 'application/json'},
      body: JSON.stringify({
        email: formData.get('email'),
        password: formData.get('password')
      })
    })
    const result = await res.json()
    console.log(result)
}

I checked everything and found nothing.