It could be that your account doesn't have the necessary permissions or that the IGTV video doesn't meet the sharing criteria. Also, make sure your Instagram app is updated to the latest version.
There are multiple potential reasons. One is that your IGTV might not be fully processed or approved yet. Instagram may have certain checks and balances in place before allowing sharing to stories. Also, if you have a new account, there could be some limitations on sharing until your account gains more credibility. Additionally, if you have violated any of Instagram's community guidelines in the past, this could restrict your ability to share IGTV to story, even if the current IGTV content is compliant.
It might be that the specific IGTV video you're trying to share has some restrictions or is not compatible with being shared on stories. Also, ensure your app is up to date as older versions might have sharing issues.
It might be because the IGTV video you're trying to share doesn't meet the platform's sharing criteria, or your internet connection is unstable. Try again later or check for any platform updates.
It could be that your account doesn't have the necessary permissions or there's a temporary issue with the platform. Sometimes, updates or server problems can cause this kind of sharing issue.