There could be several reasons. Sometimes it's a bug in the Instagram app. Another possibility is that you've been blocked by the person or your account has violated some of Instagram's rules and lost the ability to reply for a while.
Maybe your app is glitching or there's a connection issue. Try restarting your device and the app.
It could be that the person has restricted replies to their story, or you don't have the necessary permissions. Also, check if your Instagram account is up to date.
Maybe your app is having glitches. Try restarting it and see if that helps.
Maybe there's a technical glitch or your settings are off. Check your app permissions and internet connection.
Maybe there's a technical glitch or a problem with the platform. Sometimes these things happen randomly.
Maybe there's a technical glitch or your connection is unstable. Try refreshing the page or checking your internet settings.
It could be that the person has restricted replies to their Snap story, or your account might have some settings issue. Check both your and their settings.
Sometimes, if your account has violated IG's terms of use, your reply function could be temporarily disabled. Also, make sure you have the latest version of the app installed.
It could be that the person who posted the story has limited who can reply. Sometimes, they might only allow certain friends or followers to do so. Also, there could be technical glitches on the platform.
Maybe you don't have the permission or the feature isn't available for that particular story. Sometimes, it could be a glitch in the app too.
Maybe it's a technical glitch. Sometimes Instagram has these random issues that get fixed after a while.
Maybe they don't have the permission or the feature to do it. Sometimes, your settings might prevent others from reposting.