Facebook only allows one application to have a specific 'namespace' or Canvas Page name. The Namespace is used for the URL to visit your app at on Facebook, like:
apps.facebook.com/joomla-extensions/
It's used for a lot of other things that are unique to your application as well.
So, if you already have an app created with the namespace/Canvas Page name you like, you'll need to rename that app's canvas page name so you can use it in the new app.
Hope that helps explain,
Alex
The Canvas Page URL you requested is already taken.
