I followed the AWS documentation How IAM Users Sign In to Your AWS Account to sign in IAM with the link,
https://username.aws.amazon.com/console
I don't know what's the Account? I tried to fill my root account (an email), but failed to log in.

Go to IAM dashboard. You will see the sign in link there. Use that URL to sign in. When you use that URL, Account will be pre filled.
https://AWS-account-ID-or-alias.signin.aws.amazon.com/console - It is the account ID not the user ID.