It could be that your internet connection is unstable. A weak connection might cause problems with sharing on IG. Also, check if your account settings are allowing sharing of posts to the story feature.
It could be that your internet connection is unstable. Poor connection can stop you from sharing on IG story. Also, check if you have the latest version of the app installed; outdated versions might have bugs.
There might be an issue with the post itself. For example, if it violates IG's community guidelines or has some incompatible format, it won't be shareable to your story.
It might be that your account has some restrictions or violates IG's terms of use. Another possibility is that the post you're trying to share doesn't meet IG's content guidelines.
Sometimes, IG has temporary technical issues. Wait for a while and try again later. Also, check if you have the latest version of the IG app installed as older versions might have sharing glitches.