I'm doing a client-side form validation while setting the custom warning messages, but that same pop-up message keeps showing even when the input is filled with the name.
For example i have an input field to get the user name on the code below:
<form id="form" name="contactForm" method="post" action="php/formulario_contactos.php">
            <div>
                <label for="name">Your name</label> 
                <input type="text" id="name" name="name" required="">
            </div>
            <div>
                <label for="mail">Your email</label>
                <input type="email" id="mail" name="user_mail" required="">
            </div>                
            <div>
                <label for="topic">Select Topic</label>
                <select id="topic" name="topic" required="">
                    <option selected disabled hidden value="">Choose a Topic</option>
                    <option value="link">Site Link</option>
                    <option value="copyright">Copyright</option>
                    <option value="errors">Site/Article errors</option>
                    <option value="feedback">Feedback</option>
                    <option value="other">Other</option>
                </select>
            </div>                
            <div>
                <label for="msg">Your message</label>
                <textarea id="msg" name="user_message" required=""></textarea>
            </div>                
            <div class="button">
                <button type="submit">Submit</button> 
            </div>
        </form>
And i wrote the javascript code below to change the default browser error message to the one below:
var inputName = document.getElementById('name');
var form = document.getElementById('form');
form.onsubmit = validateForm();
function validateForm() {
     if(inputName.value === ""){
         inputName.setCustomValidity("Name is required");
         return false;
     } else {
         inputName.setCustomValidty("");
     }
}
What am i doing incorrectly?
                        
There were two issues in your JavaScript that were causing your validation to break:
validateFormfunction was running on page load, rather than when the user submits the form. I've added in a revised event handler to resolve thissetCustomValidtyshould besetCustomValidityThe following code fixes those issues.
HTML:
JavaScript: