First, check the post's privacy settings. If it's private and not shareable, there's not much you can do. But if it's a public post, try refreshing the page. Sometimes a simple refresh can make the share button work. Also, make sure your Instagram app is up - to - date. Outdated versions may have bugs that prevent sharing.
Sometimes, it's because of an issue with your connection. If your internet connection is weak or unstable, the sharing function may not work properly. Ensure that you have a good and stable connection. Also, if the original poster has disabled the share option for their post, you won't be able to share it to your story.
There could be an issue with the post's format. For instance, if it's a very old post or in a format that Instagram no longer fully supports for sharing to stories. It could also be related to your device. Maybe your device is having problems with its connection to Instagram's servers, or there are some software glitches on your device that are interfering with the sharing function.
One possible reason is that the post owner has restricted sharing. Some people set their posts to be private or not shareable to protect their content. Another reason could be a technical glitch on Instagram. Try restarting the app or checking for updates.
It could be that your account settings are restricting the sharing feature. Check your privacy and sharing settings in Instagram to make sure everything is enabled correctly. Also, make sure the post you're trying to share is allowed for sharing to stories by the original poster.