Seber Cart uses the Joomla message system to display error messages to the end user.  The most common place this occurs is on the Shipping/Billing details page.  If your template does not have the message include then the error messages will not be displayed if a user doesn't fill in a required field for example.

Make sure you template has the following code:

<jdoc:include type="message" />

This is usually placed just before the component in the template. 

If you are not sure where to do this, just place the above code in front of <jdoc:include type="component" /> in your index.php file for the template.