Turn off Portrait Orientation Lock in Control Center
Swipe down from the top-right corner of the screen
Tap the rotation lock button if it is enabled
Make sure the app supports screen rotation
Rotate the iPad physically to landscape or portrait
Restart the iPad if the screen still does not rotate
Check for iPadOS updates
Disconnect the iPad from any keyboard or accessory that may lock orientation
If the issue continues, reset all settings in Settings > General > Transfer or Reset iPad > Reset > Reset All Settings