I can't get sound in Reaper. Sound registers on my interface but it doesn't seem like it wants to connect. When it would work it would say connecting Apollo Twin when I turned it on, now it just does a beep.
I haven't used Reaper in years so I doubt I have the answer, but have you checked the Audio > Device settings to make sure it's seeing your interface, using the correct driver, set for the right number of inputs, etc?
What the poster above said. Also make sure you haven't routed anything incorrectly, that the correct input is selected on the target track, that it's armed etc. Could be a number of things.