I have made an amp-story which, when visited for the first time asks for "You must enable the amp-story experiment to view this content".
This is the link to the story: https://www.krenovate.com/amp/campaign/pets-completed.html
How can I get rid of that prompt?
This is seen when using version 0.1 of amp-story. You can resolve this by using version 1.0. The version is dependent on the URL of the amp-story script you include in your document. For 1.0, you want:
<script async custom-element="amp-story" src="https://cdn.ampproject.org/v0/amp-story-1.0.js"></script>