Hi,
There are two volume involved here, which might make it a bit confusing.
The first is the Directory volume, which is a description of the nodes and volumes in the project. This volume can't be built (althogh you can build nodes from it, but the volumes of this nodes has to be built first). As i understand you have configured the volume volmydemo with a RootVolumeConfig object here.
The second volume is the RootVolume 'volmydemo', which can be opened with 'Open Volume' from the RootVolumeConfig object. Here you edit the Plant and Node hierarchy, and this is the volume you should build. But first you have to enter the OperatingSystem in VolumeAttributes for this volume, otherwise you get the NOOPSYS error. A minimal system at least has to contatin a node hierarchy with a $NodeHier and a $Node object, and I think its not until you create a PlcPgm that you have to state the OperatingSystem.