Even though your Databricks job only has one task, Power Automate might still treats it as a multi-task job under the hood. That’s why you're getting the error when trying to fetch the output directly from the job run.
Here’s a simple workaround you can try and let me know if it works:
1. Trigger the job and grab the run_id from the response.
2. Use the Get Job Run API (GET /api/2.1/jobs/runs/get) to fetch details of the run.
3. From that response, extract the run_id from the tasks block(there should be just one).
4. Then call the Get Task Run Output API (GET /api/2.1/jobs/runs/get-output?run_id=<task_run_id>) to get the actual output.