Was the activation code sent by Flexera to you, or to somebody else?

The activation code may only be usable by the person (email address) it was sent to, so if that isn't you then that could explain the problem you're having. If you want to access the system using a different email address from where the code was sent to then you may need to contact your Account Manager to ask for a code to be sent to the email address you want to use.