Solution: The ArcWeb Explorer JavaScript to Flex Bridge library is missing from your application. The URL for the current version is http://api.arcwebservices.com/awx/v4/awxbridge-4.0.1.js. See Getting started with ArcWeb Explorer JavaScript to Flex Bridge for details.
Solution: There are many possible causes for this error message. For example, you might not have registered the correct API Key for this Web site. See the ESRI Knowledge Base for a list of possible solutions.