Answer
By default, a REST Call task only continues automatically on standard OK-class responses; for any other status code you need to handle (like a 404 meaning 'doesn't exist yet'), explicitly add it under the task's 'Add/Modify Response Codes' so the workflow treats it as an expected outcome and continues rather than stopping. Also be aware the _Response variable is only populated when the actual response deviates from the standard OK path — if you're checking it in a gateway/condition afterward, check for its existence first rather than assuming it's always set, or the gateway itself can error out on the standard-OK path where _Response was never set.