It could be that your account doesn't have the necessary permissions or your IGTV content doesn't meet the sharing criteria. Sometimes, it could also be a temporary issue with the platform.
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 could be that your account settings are not configured correctly for sharing. Check your privacy and sharing settings within the app. Also, make sure you have the latest version of the app installed as older versions might have this limitation.
It could be that the platform has certain restrictions or your account doesn't have the proper authorization. Also, make sure you're following the correct sharing steps.