I think there are a couple of things here.
Firstly, you are trying to share from your local computer, which won’t be available for LinkedIn to read and share. You need to deploy to the Cloud or another production environment.
Secondly, unless you want to to only share the homepage of your app, you’ll need to create a Deeplink (using the Deeplink module in the Marketplace, or page URL in page properties) to the correct content you want to share from your app.
Hope this helps, good luck!