It is very likely that this feature is disabled by your Parental Controls. I would advise that you check if you have the Parental Controls active on your account. If that is the case please speak with your parents/legal guardians and ask them to enable the Real ID feature for you.
If you are an adult and you did not setup the parental controls you can contact the Account and Billing support for assistance with removing this feature. You can contact them by phone: