Intro component with sign up form using plain JavaScript

Any feedback would be appreciated:) For showing the error message, my approach is to have the element ready in HTML , then select the elements right after input fields to fill the content in the pre-existing elements:

inputfield.nextElementSibling.innerHTML = fields[i].getAttribute("placeholder") + " cannot be empty";

What do you guys think? I would love to see if there's a better way:)