1) No one here can see the admin link you provided.
2) WordPress has no “Web Story” feature. What plugin are you using for this?
I am using Google web story. Below is the one published web story, which has no poster image, though I have added. the poster image is saved in admin, but it’s not showing in preview (before publishing) and even after publish.
https://zestycrush.com/web-stories/dare-to-roam-with-lions-discover-the-wild-adventure-of-vasona-safari/
I would recommend that you contact the support forum of the plugin you are using for these web stories: https://wordpress.org/support/plugin/web-stories/
