There are a few scenarios which can explain the behavior you are seeing:
Existing Target can only be added under Project Root but not under My Computer
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 .
Real-Time Module Not Installed for the Version of LabVIEW You Are Using
Verify that the Real-Time Development is fully activated or still within the allowed evaluation time period by checking the color of the license box. If the module is active, but you suspect that the installation is corrupt, run a repair on LabVIEW Real-time. The process for repairing a module is explained in Uninstall or Repair National Instruments Software or Drivers in Windows. Instead of selecting NI LabVIEW, as mentioned in the KB, select NI LabVIEW 20xx Real-Time Module.
NI CompactRIO Drivers not Installed or Incompatible
Incorrect order of Installation
Multiple Versions of LabVIEW Installed
In case of using a CompactDAQ Real-Time
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