One simple reason could be that the post format is not compatible with Insta story. For instance, some very large or complex video formats might not be supported for sharing as a story. Additionally, if you are using a third - party app to view the post and it doesn't have proper integration with Instagram's sharing features, you won't be able to share it to your story.
It could be that your internet connection is unstable. Make sure you have a strong and reliable connection. Also, check if Instagram has updated its sharing policies or if there are any restrictions on the type of post you're trying to share.
First, check the post's privacy settings. If it's a private post, ask the owner to change the settings if possible. If it's a technical issue, try closing and reopening the Instagram app. Sometimes, just restarting the app can solve minor glitches.
It could be that your Instagram app needs an update. Try updating it and see if that helps. Also, make sure you have enough storage space on your device.
It could be that your Instagram account doesn't have the necessary permissions or your app version is outdated. Check for updates and your account settings.