Your call to:
var amountvalidation = document.getElementById('input-group-placeholder');
is where you're going wrong. You're trying to get the element by it's ID attribute, yet the element you're looking for doesn't even have an ID attribute, let alone one sporting that value.
Try this:
<div id="amountValidationElement" class="input-group-placeholder">
<input type="text" class="form-control" id="CardHolderName" name="CardHolderName" value="Mr">
<span class="input-group-addon form-control-feedback-error">
<span class="glyphicon glyphicon-remove"></span>
</span>
<span class="input-group-addon form-control-feedback-success">
<span class="glyphicon glyphicon-ok"></span>
</span>
</div>
With the following call to getElementById:
var amountvalidation = document.getElementById('amountValidationElement');