How can I add IDC user as an approver in approval rule template in Code Commit

Hello.

This can be set by using "Fully qualified ARN".
https://docs.aws.amazon.com/codecommit/latest/userguide/how-to-create-pull-request-approval-rule.html

Sign in to the AWS account where CodeCommit is located using the SSO user.
Then open CloudShell and run the following command to check the "Fully qualified ARN".

aws sts get-caller-identity

When you run the command, you will receive a response like the one below, so by setting "Arn" to "Fully qualified ARN", you can use it as an approver for approval rules.

{
    "UserId": "AROYYYYYYYYYYYYYY:riku_kobayashi",
    "Account": "111111111111",
    "Arn": "arn:aws:sts::111111111111:assumed-role/AWSReservedSSO_AdministratorAccess_yyyyyyyyyyyyy/riku_kobayashi"
}