It could be that your account settings are not allowing story sharing. Check if there are any restrictions or permissions you need to enable. Also, sometimes IG updates can cause temporary issues.
Maybe there's a glitch in the IG app or your internet connection is unstable.
It might be that you haven't met the requirements for sharing stories on IG, like having a certain number of followers or not violating their community guidelines. Another possibility is that your device's software is outdated and causing compatibility problems.