In terms of the rules, yes that's fine! I'm not exactly sure what you mean by scenario as that could mean various things, but yes you can submit that.
As for how to submit, when you create a project on itch, you can upload a file that's text only. You can definitely upload .txt, .pdf, and .md files. I would assume you can upload MS Word files too. Then you submit that itch project to the jam.
I'd add a short description in the project description about what it is - is it a script you're planning to add to a game later, is it for a hypothetical game just to showcase your writing, is it something that stands alone like a story, etc., just so people know what they're reading.