There could be several reasons why you are unable to reply to Instagram Stories. Here are some possible explanations:

1. Privacy settings: The person who posted the story may have set their privacy settings to only allow replies from their followers or a selected group [1]. If you are not following them or don't meet their criteria, you won't be able to reply.

2. Story expiration: Instagram Stories have a limited lifespan and disappear after 24 hours, unless saved as highlights [1]. If you try to reply to an expired story, you won't be able to do so. Make sure you are attempting to reply within the 24-hour window from when the story was originally posted.

3. Story type: Not all types of Instagram Stories allow direct replies. If the user posts a story as a poll, quiz, or sticker, it may not be possible to reply directly to that story [1]. These interactive features are designed for users to engage differently, such as voting or answering questions. In such cases, you can engage by interacting with the story feature itself, like participating in the poll or quiz.

4. Account restrictions: If you have recently created an Instagram account or have been flagged for violating Instagram's terms of service, your ability to reply to stories could be restricted [1]. Instagram has measures in place to combat spam, harassment, and other forms of misuse. These restrictions can temporarily limit certain functionalities, including the ability to reply to stories. Ensure that your account is not subject to any limitations. If you believe your account has been wrongly restricted, you can contact Instagram's support team for assistance.

5. Technical glitches: Like any platform, Instagram is not immune to technical glitches and bugs. Issues or updates could be implemented that affect the platform's functionality [1]. If you can't reply to someone's story, it could be a temporary glitch that will resolve itself with time. In such cases, wait for a while or try again later. You can also try closing and reopening the Instagram app or clearing the app cache to see if that helps.

6. Device and app version: Sometimes, the inability to reply to someone's story could be traced back to the device or app version you are using [1]. Make sure to keep your Instagram app updated to the latest version to ensure access to all features. If you are using an older or unsupported device, you may encounter issues affecting your ability to reply to stories. Consider updating your device or trying a different device to see if that resolves the problem.

Learn more:

1. Why Can't I Reply to Someone's Story on Instagram?
2. How to control who can reply to your Instagram story - India Today
3. Solved: I can't reply story in instagram with my samsung a7 phone - Samsung Community