The original poster can delete their Story at any time. If you're trying to view a Story that isn't 24 hours old yet but was deleted by the person who created it, you'll see the "This Story is Unavailable" message.[1] X Research source.
What Does This Story Is Unavailable Mean on Instagram: 6 Reasons Why It Shows. 1. The Person Who Posted the Story Just Deleted It. 2. The Story Recently Expired After Meeting the 24-Hour Time Limit.

Moving forward, it's essential to keep these visual contexts in mind when discussing This Story Is Unavailable.
The This story is unavailable or Story Unavailable means the story you were trying to view is inaccessible at the moment. This message usually shows up when you try to view a story that has expired as they are only available for 24 hours.

Instagram Story Unavailable: Causes and How to Identify Each Issue. Seeing "Story Unavailable" on Instagram doesnt always mean youve been blocked. Since Instagram doesnt notify users about blocks, youll need to look for specific clues to figure out whats really going on.

Among the many Instagram glitches and errors, the one that frustrates users the most is the pop-up: This story is unavailable. It really leaves us curious as to what we missed out on. A gossip-worthy story of those ex-lovers reuniting?
The "This Story is Unavailable" message on Instagram often signals that a story has expired, been deleted, or is restricted due to privacy settings.
Instagram story unavailable may also be caused by the app glitches. You can try restarting the app to fix it and you can also check if Instagram is down. Cause 10: Unstable Internet Connection.