Sorry to hear you are having issues. From what I understand, Fences from steam will never ask for activation key. If activation was asked for its from another version of fences, possibly the trial version of fences. Please make sure to remove all version of fences before install the steam version.
What you can try is:-
1) Do the purge steps by following these steps : https://forums.stardock.com/486084/fences-support-faq#reinstalling make sure to do the last part on that steps with regard on Steam version (still need uninstall, even after the purge part)
2) Reinstall Fences within Steam. Do not install any other version. try these steps : https://forums.stardock.com/486311/accounts-and-activation-support-faq#steamkey
Stardock Community Assistant.