The most common cause of errors when adding a database to CodeGuard occurs because our servers are not able to connect to your database host. For security, your hosting provider and/or database host may block all external connections, but in most cases, you are able to specify a list of allowed hosts. This process, called "whitelisting", must be done prior to adding your database to CodeGuard. See the Whitelisting CodeGuard's IP Addresses article for details on what addresses should be added or this video tutorial for a walkthrough of this process.
The process for doing this will vary with different hosting providers, so you may need to consult their support documentation for more specific instructions.