The sleeper has to be bottomless for you to woohoo them on the couch. You have to set the CASP settings on the clothes that the sleeping sim is wearing. Click on the sleeper, choose "Kinky World" > "Set CASP Properties". Then set some value for the items the sim is wearing. The bottom item should be something that can be pulled down, I think, or possibly something that can remain on during woohoo. After setting the clothes, then low shark the sleeper. Then you'll be able to woohoo them.