Hey!
Strawberry Starter wasn't really designed for this, sadly. When you upload a project with multiple html files on itch, it gets hosted on itch's content delivery server (html-classic.itch.zone). What that means is that if you have any absolute paths in your project (starts with a "/", e.g. "/post/folder/whatever.md") they'll be broken. You need to use relative paths (e.g. "../../post/folder/whatever.md").
Basically you'd have to edit the 11ty template itself and remove all the absolute paths, probably by writing a filter for all the links. Sorry!