As I have answered in PM, and as Andra has answered above, Windows Task Scheduler execution is no longer supported since 2017.1 version. So the only option for Unattended without Orchestrator is to run the process manually and then lock the screen.
You already said that it works to run the Unattended Robot in uiautomation process if Simulate type is checked. If Simulate type is not checked it with throw an error (because as stated in the Activities user guide, Simulate type/click option is for background automation - that means that it interacts in the background and you as a user can do other things in the meantime, without losing the mouse/keyboard control, even if it means locking the screen).
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