Configuration mix control

There is no loopback feature on the Scarlett 18i20, so you will not be able to record the output of your computer internally. The only way you would be able to do this would be by looping a pair of cables from the outputs on the 18i20 back to a pair of the inputs and then record these inputs to a track in Reaper. You will need to make sure that the track that you are recording to is either muted or has no output set to it, otherwise you will setup a feedback loop.