I canot signin as IAM user.

Hello.

Did IAM users originally have access to the management console?
If you were able to access it originally, try resetting the IAM user's password.
If you can sign in as the root user, you can reset the IAM user's password.
https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_passwords_admin-change-user.html

Also check that the IAM user has console access enabled.

At the point you create the IAM user, you've the option to choose if the user should have access via the management console and or programmatically.

Did you choose that of management console?

Also, a password is created for the IAM user when you create this user. You've the option to choose if you want the user to change the password when they try to login the first time. You may want to confirm if you chosed this option in the first place, an that would mean creating a new password for yourself (well, except you've attempted login before with this user and you succeeded).

Hello,

Here's a step-by-step approach to diagnose the issue:

• Verify Credentials: Double-check your IAM user's username and password.
• Console Password: Log in to the IAM console with your root user credentials and ensure "Console password" is enabled for the IAM user you're trying to access.
• MFA: Check if MFA is enabled for your IAM user and use the appropriate code if necessary.
• Login Page: Confirm you're on the correct AWS sign-in page for IAM users.
• IAM Identity Center: If your organization uses IAM Identity Center, access the AWS access portal with your organizational credentials.

https://docs.aws.amazon.com/signin/latest/userguide/troubleshooting-sign-in-issues.html