We got this issue when trying to restore a site collection backup file in a web application.
The operation that you are attempting to perform cannot be completed successfully. No content databases in the web application were available to store your site collection. The existing content databases may have reached the maximum number of site collections, or be set to read-only, or be offline, or may already contain a copy of this site collection.
The reason for this error is that we have restored the backup file already in a different site collection in same web application. We got this error when trying to restore the same backup file in different site collection in same web application.
SharePoint 2010 by default accepts a backup file to be restored only once for a content database. Hence, whenever we are trying to restore the backup file to different site collection in same content database, we got this error.
The Resolution for this issue is to restore the backup file in different web application. This makes sure that the backup file is restored in a new content database.