Go to Master Account IAM -> Policies -> Create Policy 

Select "Create Your Own Policy"

Add title, description and Policy Document

{
    "Version": "2012-10-17",
    "Statement": {
        "Effect": "Allow",
        "Action": "sts:AssumeRole",
        "Resource": "PASTE ARN HERE"
    }
}

After creating policy, find it in policies list

And attach to IAM-Manager Lambda Role