New IAM user for AWS console

When signing in to the AWS Management Console as an IAM user, you need to provide three pieces of information:

1. Your AWS account ID or account alias
2. Your IAM user name
3. Your password

The "account name" that's being requested is actually your AWS account ID or account alias. This is required because IAM users exist within a specific AWS account, and AWS needs to know which account to authenticate against.

Your AWS account ID is a 12-digit number (without hyphens) that uniquely identifies your AWS account. Alternatively, if your organization has set up an account alias (a more friendly name), you can use that instead.

The sign-in URL for IAM users typically looks like: https://[your-account-id-or-alias].signin.aws.amazon.com/console/

When your administrator created your IAM user, they should have provided you with this account ID or alias along with your sign-in credentials. If you don't have this information, you'll need to contact your AWS account administrator to get the correct account ID or alias for signing in.
Sources
How IAM users sign in to AWS - AWS Identity and Access Management
Unable to Sign in to AWS Management Console with New User | AWS re:Post