It could be that your account doesn't have the necessary permissions or doesn't meet the requirements for adding IGTV to your story. Check the app's settings and terms of use for more info.
Maybe the IGTV format isn't compatible with stories. Check if there are any specific requirements for adding content to stories. For example, some platforms have restrictions on the type of media, its size, or aspect ratio.
There could be several reasons. Maybe the video format isn't supported, or there's a limit on the type or size of videos you can add. Sometimes, it's just a temporary bug on the platform. Wait a while and try again later.