1.The main reason for this error is installing the robot in user mode.(Robot - Robot Service) instead of service mode. If you install the robot in user mode it does not have capability to run the process in un attended mode and in the first ui element action it will fail saying that cannot bring application to foreground and the windows session is locked so bot don’t have capability to make rdp connection to the Vm.
Solution would be select the un attended robot option while installing the studio and in advanced options you can check the check box to enable studio to install the bot in service mode. Thanks
To install in Service Mode, you need to run the installer with an Administrator user and in the first window choose Custom → Unattended
More details here(Robot - Installing the Robot)