Hello Everyone. Can anyone help me how to do form error message after submit button, Here my codes I tried but i couldn't get.

Can anyone help me how to do form error message after submit button, Here my codes I tried but i couldn't get it.

 <form method="POST"  >
                    <h2>Contact Us</h2>
                    <label>Name</label>
                    <input type="text" name="contactName" class="form-control" placeholder="e.g. John Lim" required  />
                    <br />
                    <label>Email</label>
                    <input type="email" name="contactEmail" class="form-control" placeholder="Email" required />
                    <br />
                    <label>Phone Number</label>
                    <input type="tel" name="contactNumber" class="form-control" placeholder="e.g. +60 11111111" required />
                    <label>Description</label>
                    <textarea class="form-control" type="text" rows="5" cols="5" name="contactDesciption" placeholder="Briefly describe the bespoke talent you wish for us to identify for you in 200 words." required> </textarea>
                    <button class="btn" type="submit" formmethod="post">Request Talent</button>
                </form>


input:required {
    border: 1px solid red;
}
input:valid {
    border: 1px solid black;
}