Why sending 'true' not helps? The value of the 'Access-Control-Allow-Credentials' header in the response is '' which must be 'true'

Got this error:

The value of the 'Access-Control-Allow-Credentials' header in the response is '' which must be 'true' when the request's credentials mode is 'include'.

Here when pressing Login button in top right. https://sp-poc.com/admin

This code is called:

axios
  .post(
    `loginEmail`,
    {
      headers: {
        crossDomain: true,
      },
    },
    { withCredentials: true }
  )

backend is on a different domain, but I did set DNS record: A and CNAME record for it as it is suggested here: https://vercel.com/support/articles/pointing-subdomains-to-external-services

I event tried to set Access-Control-Allow-Credentials manually in response.

func loginEmail(_ req: Request) throws -> Response
{
    let response = Response(status: .ok)
    let cookie = HTTPCookies.Value(string: "abcdef", isHTTPOnly: true)
    response.cookies["userId"] = cookie
    response.headers = HTTPHeaders([("Access-Control-Allow-Credentials", "true")])
    return response
}

What is wrong here?

1. I though if I do the A, CNAME record trick, it will not seem as a cross-site request.
2. If it seem a cross site request why returning true does not help?