Edit the Trust Policy:

Go to the IAM console in the parent account and locate the role that the EC2 instance needs to assume.

Edit the trust policy of the role to ensure that it correctly allows the sts

action for the role ARN in the child account.

Update the Role ARN:

Verify that the target role ARN specified in the trust policy matches the role ARN created by the CloudFormation stack in the child account.

If necessary, update the ARN to reflect the correct role in the child account.

Save and Test:

Save the updated trust policy and ensure there are no syntax errors.

Test the setup by attempting to assume the role from the EC2 instance in the child account. Verify that the instance can successfully assume the role and perform the required actions.

This ensures that the EC2 instance in the child account can assume the role in the parent account, resolving the permission issue.

References

AWS IAM Documentation on Trust Policies【51】.