Feature Modeling Parameter sets - "Add Variable(s) Failed'. Selecting the Add Variables button in the Parameter Sets dialog box, results in the message "Add Variable(s) Failed".
If all of the Global Variables that you created are already listed in the columns of the Parameter Sets dialog box, this message will appear. To verify this, simply add a new Global Variable and then hit the Add Variables button. A separate box will come up, allowing it to be added to the columns listed.