On my Admob account, it says that, "AdMob account inactive Your AdMob account has been inactive for over 5 months. Accounts that haven't served ads for 6 months will be automatically deactivated." Then a Learn More button. It says there that there is a checkbox but it is nowhere to be found.
The message I have received is just a warning. After I exceeded the 6 months, that's the time that I can click the checkbox. As long as it won't appear means your AdMob is still active.