There are a few scenarios which can explain the behavior you are seeing:
First of all, make sure that you are trying to add an existing target under the correct level in the project explorer. Stand-alone RT controllers need to be added under the Project: root and not under My Computer as shown in the picture below and explained in Adding RT Targets to the LabVIEW Project .
If the Targets and Devices... option is missing when you right-click on the project root in the LabVIEW Project, it might be due to an issue with the LabVIEW Real-Time Module.
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