In order for us to create a tip button and do member management, you need to have your Ghost site connected to an active Stripe account. You should have membership turned on, and at least one paid tier set up if you want to use the full capabilities of the Autoresponder. You do not have to charge for your site in order to use Outpost, just have a tier set up.
If you are hosted on GhostPro, you need to be on a plan that allows for custom integrations, which is currently the Creator plan and above.