If you continue to have this problem after several attempts, you can temporarily disconnect your computer from the internet to use the web- or phone-based activation methods instead. These options will be automatically offered if your computer doesn't have internet access during activation.
This usually means that you entered the correct serial number and the software is trying to activate, there's something preventing the activation from getting through as expected. Typically, this is a problem with your internet firewall, proxy server, or other network security. It might also indicate a temporary problem with your internet connection.