jQuery(function($) {

	var validation_holder;

	$("form#send input[name='submit']").click(function() {

	var validation_holder = 0;


		var username 			= $("form#send input[name='username']").val();
		var password			= $("form#send input[name='password']").val();

		/* validation start */

		if(username == "") {
			$("span.val_username").html("This field is required.").addClass('validate');
			validation_holder = 1;
		} else {
			$("span.val_username").html("");
			}


		if(password == "") {
			$("span.val_password").html("This field is required.").addClass('validate');
			validation_holder = 1;
		} else {
				$("span.val_password").html("");
			}


		if(validation_holder == 1) { // if have a field is blank, return false
			$("p.validate_msg").slideDown("fast");
			return false;
		}  validation_holder = 0; // else return true
		/* validation end */
	}); // click end

}); // jQuery End




<div id="popup-log" class="popup">
  <div class="popup-content">
                <p class="validate_msg">Please fix the errors below!</p>
    <div class="event-header">
      <h6>PRISIJUNGIMAS!</h6>
    </div>
      <form id="send" method="post"">
        <div class="form-group">
          <input type="text" placeholder="Prisijungimo vardas..."><span class="val_username"></span>
          <label for="name">
            <i class="fa fa-user"></i>
          </label>
        </div>
        <div class="form-group">
	<span class="val_password"></span>
          <input type="password" placeholder="Slaptažodis..."><span class="val_password"></span>
          <label for="password">
            <i class="fas fa-lock"></i>
          </label>
        </div>
        <button type="button" name="submit" class="main-btn-rect" value="Prisijungti">
          <i class="fa fa-paper-plane"></i>Prisijungti</button>
      </form>
    <span class="fade-out main-btn-circle">╳</span>
  </div>
</div>