There might be some rules or policies of the platform that are preventing you from sharing public stories. For example, you might not have met certain requirements like having a verified account or following specific community guidelines. Take a look at the platform's terms of use for more clarity.
Maybe there's a technical glitch or a bug. Try restarting the app or your device and see if that helps.
It could be that your account settings are restricting the sharing of public stories. Check your privacy and sharing settings to make sure everything is configured correctly. Also, make sure you have the latest version of the app installed.