I see you posted another post with another error. I replied to that post that your installation was broken and to uninstall, reinstall.
Try that first. If you still get this error after doing that, try this:
The issue is caused by corrupt mysql table. Here is the fix:
Use phpMyAdmin to check if the mysql table wp_visitor_maps_wo got corrupted. If it is crashed or corrupted: first deactivate the plugin from the WP admin, then use phpMyAdmin to select the table and select repair. When the repair is complete, re-activate the plugin and it should work again.