Every express VI (and many of the other VIs) has an error in and an error out. These are used to pass error codes through your program, so that if something goes wrong you have a clue of where the problem is. This is especially important when communicating with external devices, which we arent doing today. Connect the error out of the Simulate Sig to the error in of the Spectrum, and then create an indicator for the error out of the Spectrum. If there were any errors, a message would be displayed in this indicator. Run your program and verify that no error is displayed.