When you see 'this can't be shared to your story', it implies that the content has been restricted from being added to your story feature. This can happen for various reasons. It might be that the content is inappropriate according to the platform's community guidelines, so it's blocked from being shared further in stories. Another possibility is that the content creator has specifically disabled sharing to stories for their content. For instance, on Instagram, a user might not want their photo to be shared as someone else's story, so they can set the appropriate permissions to prevent this.
It simply indicates that you are not allowed to share it in your story. Maybe it's copyrighted material and the owner doesn't want it spread in that way. Or it could be that the platform has determined it's not suitable for story sharing based on its own rules and algorithms.
Well, 'this can't be shared with your story' might imply that there is a particular element which has certain restrictions. Maybe it's a piece of personal or private knowledge that should not be part of a public or told story. It could also be something that is not in line with the narrative style or intended audience of the story.
You might be seeing this because the content is not meant to be spread further in the form of a story. It could be that the source has restricted such sharing.
Figuring out if your story is shared can be tricky. Some websites or apps might give you stats, but not all do. And even then, it might not cover all the possible ways it could have been shared elsewhere.