You see a failed tracking request with this error message when an invalid idSite is configured as part of your tracking code.

For example, in the default JavaScript tracking code the idSite is configured like this:

_paq.push(['setSiteId', '1']);

When you get this error, make sure your tracking code includes this line (or for example tracker.setSiteId(idSite) if you configure the tracker manually). Then check the configured ID actually exists by going to “Administration” => “Measurables/Websites”. Maybe the site has been deleted, or never existed in the first place and setting the correct ID will resolve the problem.