There could be a few different reasons this may occur but here are the most common.
1. The content owner could choose to block certain geographical regions from viewing the content.
2. The video could be pending verification.
3. The video could have been flagged for review.
4. The video could have been deleted or removed.
You can visit this link to find out more about Geo-blocking https://www.pornhub.com/blog/geo-blocking-explained