not possible to post insta story

Unfortnately, not using Adobe Express. I believe it's a limitation imposed by Instagram themselves, if I remember well.
If that helps, you can create a new video, but that is a static one - ie: no animation whatsoever - and download it as a MP4. it will appear as a static post, with the added perk of being the exact lengh you want, instead of the very short standard lengh of a real static story (if that makes sense)

You can start with a template for a video, or simply choose "instagram reel" in the preset...(same size as a story, but set up as a video)

Hope it helps!