aws sts get-caller-identity lists the root account i am unable to switch to iam user by using AWS CLI

Review Configure the AWS CLI for the settings that the AWS Command Line Interface (AWS CLI) uses to interact with AWS.

Some options to select the IAM user:

• use the AWS CLI --profile parameter (see Using named profiles )
• set the AWS_PROFILE environment variable as the default profile

Additionally, review Security best practices in IAM and look at using temporary credentials when accessing AWS through federation with an identity provider.

Do you have aws credentials correctly configured? Typically is a good practice to assume role https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_use_switch-role-cli.html so that you always work with temporary credentials