How to lock screen horizontally or vertically on an ipad

Then you have set the side button to control rotation lock. You can lock the rotation by moving that button to the down position.

If you prefer to use the side button for mute, then go into Settings > General > Use Side Switch to ..

When the side switch is set to control mute, then the lock rotation icon will appear in the control centre ("short little area in grey") in the place that the mute appears now.

Read this.

Swipe up from the bottom of the screen to reveal the Control Center. Tap on the Orientation lock icon. That's the circular arrow around a small lock.