bootstrap 4 input error
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" integrity="sha384-rwoIResjU2yc3z8GV/NPeZWAv56rSmLldC3R/AZzGRnGxQQKnKkoFVhFQhNUwEyJ" crossorigin="anonymous">
<!--
Title: bootstrap 4 input error
-->
<!--Go to https://www.w3schools.com/bootstrap4/bootstrap_forms.asp for more info-->
<form action="/action_page.php" class="was-validated">
<div class="form-group">
<label for="uname">Username:</label>
<input type="text" class="form-control" id="uname" placeholder="Enter username" name="uname" required>
<div class="valid-feedback">Valid.</div>
<div class="invalid-feedback">Please fill out this field.</div>
</div>
<div class="form-group">
<label for="pwd">Password:</label>
<input type="password" class="form-control" id="pwd" placeholder="Enter password" name="pswd" required>
<div class="valid-feedback">Valid.</div>
<div class="invalid-feedback">Please fill out this field.</div>
</div>
<div class="form-group form-check">
<label class="form-check-label">
<input class="form-check-input" type="checkbox" name="remember" required> I agree on blabla.
<div class="valid-feedback">Valid.</div>
<div class="invalid-feedback">Check this checkbox to continue.</div>
</label>
</div>
<button type="submit" class="btn btn-primary">Submit</button>
</form>