I can see the instance from my linked account in billing console. but I can't find where is that instance. is it possible to access linked account's instance from root account in AWS.
Unfortunately this is not supported functionality within AWS according the documentation.
You can only switch roles when you sign in as an IAM user. You cannot switch roles if you sign in as the AWS account root user.
You will need to create an AWS IAM user with the permissions of "Sts:AssumeRole" for the other account you would like to connect to.