shut down the account

Hello.

First, is it possible to sign in to your AWS account as the root user?
You must be able to sign in as the root user to close your AWS account.
Root users can sign in by following the steps in the document below.
https://docs.aws.amazon.com/signin/latest/userguide/introduction-to-root-user-sign-in-tutorial.html

Once you are able to sign in as a root user, you can cancel your AWS account by following the steps in the document below.
Your AWS account will be permanently closed and you will no longer be able to use it 90 days after you perform this operation.
https://docs.aws.amazon.com/accounts/latest/reference/manage-acct-closing.html#close-account-procedure