There could be several reasons. Maybe there are restrictions set by the platform due to privacy policies, or it might be a technical glitch. Another possibility is that your account doesn't have the necessary permissions.
It might be because of a recent update to the platform that has caused some sharing functionalities to be temporarily unavailable. Wait for a while and see if it gets fixed.
Sometimes, it might be due to a temporary server issue on the platform. Just wait for a while and try again later. Also, make sure you have the latest version of the app installed.
Ensure that you are not violating any community guidelines with the reel content. If the content is inappropriate or against the rules, sharing will be restricted. Also, try restarting your device. Sometimes a simple restart can clear up any glitches that are preventing the sharing function.
It could be that your account settings are restricting the sharing feature. Check your privacy and sharing settings to make sure they allow for sharing reels to your story. Also, make sure you have the latest version of the app installed.