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 to Stories isn’t available to everyone.
To share someone's post from Feed to your story:

Instagram Lite app for Android
- Tap
below the photo or video in Feed. - Tap Add post/video to your story.
- Tap Add to Story.
Keep in mind that when you share someone's post to your story, anyone who views it can see who originally posted it and can view the original account.
Keep in mind that when you share someone's post to your story, anyone who views it can see who originally posted it and can view the original account. Learn how to
share a shop to your Instagram Stories.
If you have a public account, you can turn off the ability for others to re-share your posts.
To stop allowing others to re-share your posts:

Instagram app for Android and iPhone
Tap

or your profile picture in the bottom right to go to your profile.
Tap

in the top right, then tap
Settings and privacy.
- Tap Privacy, then tap Story.
- Tap
next to Allow sharing to story.

Instagram Lite app for Android
Tap

or your profile picture in the bottom right to go to your profile.
Tap

in the top right, then tap
Settings and privacy.
- Tap Privacy, then tap Story.
- Tap
next to Allow Resharing To Stories.