Maybe there's a technical glitch or your account settings are restricting it.
Well, there are several reasons. Maybe your Instagram version is outdated, or you don't have the necessary permissions. Another possibility is that the story owner has specifically disabled sharing for that particular post.
Maybe it's a privacy setting issue. The person whose post you want to share might have restricted sharing options.
Maybe they have restricted their story to certain people or groups, and you're not included.
Sometimes, if your account has violated IG's terms of use, your reply function could be temporarily disabled. Also, make sure you have the latest version of the app installed.
Maybe there's a problem with your internet connection. Check if it's stable.
Maybe there's a glitch in the IG app or your internet connection is unstable.
Maybe they have restricted who can view their stories or set their account to private.
Well, in most cases, if the Instagram user has not set any restrictions on their story sharing, you are allowed to share it. But keep in mind that some people might not like their stories being shared without permission even if the settings allow it, so it's always polite to ask first. Also, if there are any copyright or intellectual property issues involved, that could be a problem too.
It could be that the platform you're using doesn't allow sharing of certain types of stories. Or, there might be copyright or terms of use preventing it.
Maybe there are privacy settings or restrictions preventing you from sharing it.
It could be that your IG app is not updated. Try updating it and see if that helps. Also, sometimes IG has glitches and you might have to wait for them to fix it.