In most cases, this case is either a temporary issue with the connection to the license server during installation or a mistyped activation code.
To confirm the license's validity, check the license in the Customer License Portal.
If the license you entered is valid and not mistyped, ignore the message during installation. The installation can be completed without the license entered. Once SMEX is installed, open the ScanMail Management Console, navigate to Administration > Product License and see if the license appears correctly. You may also enter a different Activation Code in case you entered a wrong one initially.
Should the license still appear as invalid or expired, refer to KB 1105766 on how to troubleshoot and collect log files.