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 are several reasons this might be happening. One possibility is that your account doesn't have the necessary permissions. Another is that the IGTV video itself has some issues, such as copyright restrictions. If it's a copyright matter, you won't be able to add it to your story. First, ensure that you own the rights to the IGTV content or that it's allowed to be shared in this way. Then, double - check all the settings related to your account and the IGTV video to see if there's something preventing the addition to the story.