You can only share someone's post from Feed to your story if their account is public and they've allowed resharing of their posts. Sharing posts from Feed
How to share feed posts to Instagram Stories · Tap the paper airplane button below the post (like you would to send a direct message) · You'll then see an option
Automatic crossposting. Once you link your Instagram account to a Facebook profile or Page, you can set up automatic crossposting to automatically crosspost
