Sometimes a request will fail. Often the reason for the failure is outside of the control of the LAS installer (like when a remote data source such as an OPeNDAP server is temporarily off-line). None the less, if a request that you are monitoring encounters an error you should receive notification that request has finished. When you go to the URL for the product you requested you should see a page like this one.
If you think the problem is transitory, you might simply resubmit the request using the link provided and get your results when the request finishes. If the problem continues to happen, use the "Email to the site administrator" link to inform the folks that run the LAS of the situation.
Hitting the "request" button will resubmit the request with debug logging turned on. Experienced users of LAS may find the information contained in the debug log to be of value in pinpointing the cause of the error.