I recently received the following message while trying to restore a site collection backup:

“No content databases are available for this operation.  Create a content database, and then try the operation again.  To create a content database, click “Content databases” on the Application Management page, select the Web application to use, and then click “Add a content database”

The issue was that I had already restored the backup on the system and in the existing content database for the web application.  Since the site collection doesn’t change GUIDs you can’t restore a site collection into two site collections in a web application that has only one content database – it has to be in a separate content DB.  Add a new content DB and the error goes away.