Hi Wross14,
Thank you for posting in Apple Support Communities! Are you able to watch videos in any other app in landscape mode? You can test this in the Photos app with videos to confirm.
If you are able to watch videos in other apps, this may be isolate to the Facebook app itself. You can check for any updates that need to be installed.
To update apps manually:
- Open the App Store app and tap on your profile in the top right
- scroll down to the available app updates
- Tap Update All to install any available updates
How to manually update apps on your Apple device - Apple Support
If this continues, you may need to reach out to the app developer for further assistance. Check out how to do so here: How to contact an app developer - Apple Support.
Have a good one!