I'm gonna guess your levels are too low, or the scale is wrong. Try the left slider. The measurement might be at the bottom. Just a guess.

 

here's a picture:

 

Never seen that one! Sorry...

 

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.

 

I think the problem is coming from my old laptop? Which is acting up.

 

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.