This CL - 
https://chromium-review.googlesource.com/c/chromium/src/+/1372830/27 
adds timer support for Assistant on Chrome OS.

There is no hook to report error saying that start failed. One option is to create another method AssistantAlarmTimerController::OnTimerCreateFailed(id) and let it show the error.
https://cs.chromium.org/chromium/src/ash/public/interfaces/assistant_controller.mojom?rcl=9b931de8b79b3afeeb1ac269b4a43a8aac2a312e&l=35