If your serial number is not working try copying and pasting the serial. And be sure that you're connected to the internet.
Another reason your serial cold not be working is your DAW/Host connection is blocked. To fix this you can follow these directions:
(Windows Only) To manage the whitelist in the Windows Firewall, click Start, type firewall and click Windows Firewall. Click Allow a program or feature through Windows Firewall (or, if you’re using Windows 10, click Allow an app or feature through Windows Firewall).On the next screen, click Change settings, then untick the box next to your DAW/HOST to allow it. If you don't see your DAW/Host in the list try adding it with the Allow another app... button.
If all else fails. Open the installed standalone, run it, and enter your serial number in there and that should get you going.