Redmi note 4 screen will not rotate.

Open the toggle panel from the top of the screen, and make sure the Rotate off button isn’t turned off. This will enable the automatic screen rotation.

Please note that some screens or some app screens can’t be rotated even with the ‘Rotate off’ button turned off for example the Messaging app.

While watching YouTube screen rotate to one side but it doesn't rotate to another side.how to fix?

I'm not sure what you meant by the screen can rotate one side but it doesn't rotate to another side? Maybe you can try clearing data for the Youtube app.