This is because something has gone wrong during the installation.
At the end of the message will be a list of tables that are missing, you can use the provided link to re-run the installation routine. If that does not resolve the issue and the visitors table is the only table listed, you may want to check your MySQL version. Some older versions of MySQL (in the 5.0.x series) have issues with complex compound indexes, which we use on the visitors table. If this is the case, check with your hosting provider and see if they can upgrade to a newer version of MySQL.
If you still have issues open a new thread on the support forum and we’ll try and resolve it for you.