Very often you can't re-share other users' posts in your Instagram Stories. Here are the main causes
From posts to Stories: it is one of the most popular sharing options of Instagram, that is, the one that allows us to share a post (even not ours) in a Story. The advantage? It greatly increases the visibility for the shared post. And visibility on Instagram, as it is known, is a valuable asset.
This is also why you see many Instagram users who share in their Stories the posts of their favorite brands or accounts of companies for which they work. But unfortunately, sometimes you can't share a post on Instagram because the "Add post to your story" button doesn't appear, and there could be several reasons for that. And not all of these reasons depend on us: sometimes it's a simple problem of the app, other times it's a problem related to the account that published the post we want to turn into a Story. Other times still it's really a choice of the person who created the post in question. Here's a list of possible causes that make it impossible to share a post in Instagram Stories.
Unable to share a post in Stories: here's why
Most often if you can't share a post in Instagram Stories it's simply because the account from which it was created is private. In this case, sharing the post is always impossible. If, on the other hand, the account is public, the impossibility of sharing a post in the Stories could derive from a precise choice of the user who created it and disabled the permission to share its posts. Again, there is nothing we can do about this. Finally, it can also be a problem with our app.
Disinstalling and reinstalling the app, with a smartphone restart between the two operations, usually solves everything. But if we don't want to go that far we can also just clear the cache of the Instagram app and see what happens: on Android we have to go to Settings > Apps and notifications > Show all apps and look for the Instagram one, then tap on it and choose Storage space and cache > Empty cache.
How to prevent others from sharing our posts
If instead we are the ones who don't want our posts to be shared by others in their Stories, then we can disable sharing from within the app. Just go to Settings > Privacy > Stories and disable the appropriate toggle for re-sharing in Stories. From now on, no one will be able to share our posts in their Stories: they won't see the button anymore.