Make sure no person has logged into the VM with the robot account and then just closed the RDP window. This leaves the session active but locked. You have to SIGN OUT from the start menu. I suggest trying that first - log into the VM with the robot account, then SIGN OUT. Now run your automation and see what happens.
Are you sure the prompt is appearing on the screen? I always take and email a screenshot (using Try/Catch exception block) when there’s an error, so I can see what was on the screen.