As the title says I can't unlock System Preferences in High Sierra, however my password works for login, for authenticating app installs, and my account is an Admin when I open the Users and Groups panel. I've tried resetting my password, however it doesn't fix the issue. Every article I've seen about a similar issue the fixes don't work either as it's typically that the account isn't admin or the reset fixed it.
I found a working solution to this on this Apple support community thread, Can't install programs and unlock settings:
OSX apparently needs to regenerate a fresh auth.db from /System/Library/Security/authorization.plist.
This can be triggered in terminal with the following command: sudo mv /var/db/auth.db /var/db/auth.db.old
After rebooting the system everything works as it should be.