There could be several reasons. Maybe the reel has certain privacy settings that prevent sharing on a story. For example, the creator might have restricted sharing to only specific groups or made it so that it can't be shared outside of the app in that way. Another possibility is a technical glitch in the app. Try restarting the app and see if the option becomes available.
It might be due to a technical problem. Sometimes, there are issues with the platform's servers. Try restarting the app. If that doesn't work, check your device's internet connection. A weak or unstable connection can also stop you from sharing a reel to your story. Another possibility is that your account has been flagged for some reason, which restricts certain sharing actions.
It could be that the platform has some technical glitches or restrictions at the moment. Sometimes, these things happen and get sorted out after a while.