The first thing to try is to run the software as Administrator (when you're using a modern operating system). Secondly, make sure the serial number is added properly. Try with lower and upper case as well. If the software requires online activation, then make sure the firewall allows it to connect to the Internet. The software will display errors in case it can't connect to the Internet.
If after all these tries you still can't activate the software, contact the developer and explain the situation. A new serial might be needed to complete the operation.