Why does It show "Unavailable" in the Invite section?
The bot is only able to track invites for members who join after you setup Invite logging. For members who joined prior, we are still able to detect some information such as the Server Join Date, but the Invite code and Creator will be unknown as this data is not available to the bot. This isn't the case only for this bot, but any other bot that tracks invites.
In rare cases, it might show unavailable if the member joined with discovery, as there is no way to detect that as of yet.