Free Google administered email is going away. If you want to keep using it, you have to start paying for it. This is how to set up billing.
Go to admin.google.com.
You must sign in using your administrator account, not your personal account if they are not the same. It must be an account with the domain name you are trying to set up billing for.
If you do not see the email account for your domain name, click Use another account.
Starting from the sign-in page, enter the email address and password for your admin account (it does not end in @gmail.com).
Once you are signed in, click the Google Apps icon on the top right. It is the one that looks like nine dots. Scroll down until you see Admin. Click to go to the Google Admin console.
(If you do not see this, you may not be the Admin for your domain. Ask the person who set this up for you who is the admin. )
In the Admin console, click the menu (three horizontal lines). Go to Billing > Subscriptions.
On the Subscriptions page, click Setup Billing on the top right, then Get Started.
If this does not work or I have missed a step, please let me know.
Here are some more helpful links from Google:
I waited too long. Now what?
Follow the steps above to log in.
At least for now, everything is probably still there. Just click the button to Set Up Billing.