Lambda roles must have access to CloudWatch Logs. If you are building a policy manually, ensure that it includes:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "logs:CreateLogGroup",
            "Resource": "arn:aws:logs:region:accountID:*"
        },

        {
            "Effect": "Allow",
            "Action": [
                "logs:CreateLogStream",
                "logs:PutLogEvents"
            ],
            "Resource": [
                "arn:aws:logs:region:accountID:log-group:/aws/lambda/functionname:*"
            ]
        }
    ]
}

If the role is missing these permissions, the function still runs but it is unable to log any output to the CloudWatch service.