Your function's execution role doesn't have permission to send result to the destination

The issue is not the lambda execution policy, but you (your IAM user) does not have permissions to perform iam:AttachRolePolicy.

The reason is that the lambda will add the following service-role policy to your function execution role, regardless the fact that you already have AmazonSNSFullAccess there:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "sns:Publish",
            "Resource": "arn:aws:sns:region:xxxx:testTopic"
        }
    ]
}

You have to add the missing permissions to the IAM user you use when login to the console.