How does one enable the "Optional personal message" when creating an invitation?

There shouldn’t be any setting changes required, assuming you haven’t otherwise disabled invites. To add a personal message, you need to expand the invite modal via the “advanced” button (:gear:). Then you should see additional options for your invite including the message.

Once advanced options have been opened, you’ll see the optional personal message along with other configurable options. (note this screenshot is from a staff user, regular users will not see all the options)