That is called loopback recording, check if your audio interface can do that internally, otherwise you would need to make in using cables from the output of the mains to line Inputs of your audio interface.
Refer to your audio interface manufacturer and check if that is possible. Not a reaper issue.
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Intel core i7 6700K - socket 1151 LGA - Chipset z170 - 16G DDR4 - Nvidia GTX 1060
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