It look like a measurement of a loopback cable. Measuring a wire gives a nice flat response like that when the calibration is correct.
In REW Preferences Soundcard tab have you now selected the correct measurement channel - The one with the mic connected? The REW default is channel 2 (the right channel) for the mic, but you can change it to match your connections.
It looks like a valid measurement of a wire. That would not be the result I would expect if the program, connections, or PC was not working.
If you have gone through all the connections and REW settings and don't see a problem you could post the mdat file here and let us take a look. It may have a hint as to what is wrong or at least rule out a couple of things.