How long is the project and how long did you wait at 99%? It is normal that on long videos, it will stay at 99% for a while because it is reorganizing the file to optimize it for upload to YouTube and other places.
If you waited at least an hour, then it is unclear what the problem is. I would need to see the log. You can stop it and see if the exported file plays. Sometimes I think it can get stuck when it is closing even though it was successful.