To enable the parental control successfully you would need to follow the below mentioned steps:
1. Login to your account either in the app or website
2. Once done, click on 'menu' or 'hello' followed by 'settings'
3. Enable the toggle available against the 'parental control' option (once you enable it, the 'viewing restrictions' option would become functional
4. Now click on 'viewing restrictions' and authenticate your subscription.
5. Once you do the same, you would be led to the restrictions page. You would need to choose the category for which you would want to have the restriction enabled and thereafter click on 'save rating'
6. Now enter your pin to save the same.
Note: If you do not remember your pin, click on the 'reset your pin' option to set up a new pin. Alternatively you can use the touch ID enabled option.
This is a session specific feature, which means that for every session you would need to enter the pin only once for the first restricted content you wish to start watching.