When you installed Robot was it the community edition or enterprise? Which version did you install?
Depending on those answer should reveal what you need to do.
Prior to the latest 2021.8 Preview Community Edition (EXE) only provided a User Mode installation and in order to registered the UiRobot as a Service you needed the Enterprise Installer (MSI) and either register it as a service during installation or doing so from the command line after the fact.
It was recently announced with 2021.8 Preview Community Edition that they would start to allow the Robot in Service Mode.