Horizontal mode not work in Messages

Just fixed this exact problem on my mother’s phone. You will need to turn off zoomed display under the Settings > Display. It fixed the problem when we switched it back to standard.

Hi taiwon,

Welcome to Apple Support Communities. We understand you're unable to go in landscape mode in the Messages app. We can help point you in the right direction.

Let's make sure that Portrait Orientation Lock button is not turned on. To review that, please follow the steps outlined in this helpful article: Rotate the screen on your iPhone or iPod touch

In addition, please restart your iPhone. This can help resolve unexpected behavior. If needed, here's an article on how to properly restart your iPhone: Restart your iPhone

Please let us know if that helps.

Kind regards.