INFO: WARNING: FacesMessage(s) have been enqueued, but may not have been displayed

As to the warning referring an undisplayed Validation Error: Value is not valid message, this means that you've somewhere a <h:selectXxx> component such as <h:selectOneMenu> without an associated <h:message> and therefore JSF is not able to display faces messages about conversion/validation errors directly in the UI.

In order to fix the particular warning, just add a <h:message>:

<h:selectOneMenu id="foo" ... />
<h:message for="foo" />

Note that when you're ajax-submitting the form using <f:ajax>, then you should not forgot to include the messages in the ajax-update. To start with, use render="@form" to update the entire form.

As to the concrete validation error problem which is mentioned in the message, head to the following answer: Validation Error: Value is not valid.