I have the larger of the rain barrels on the roof spaced 5 tiles from the corner, one floor below this on the 6th tile I have placed a sink and no matter what I've done, I haven't been able to plumb the sink. I have another plumbed sink and washer near by (on different barrels) so it isn't an issue of not knowing how. as you can see in the screenshots attached I have the wrench in my inventory (equipped in hand) and the plumbing option just isn't there.
I have emptied the barrel, picked it up and replaced it and the issue persists. picking up the sink and doing the same has no effect. I have even gone as far as to build a wall in front of the fridge and testing the 3rd position the sink should be accepted in.
This particular sink is the wall hanging sinks from the bathroom of the warehouse (SW of riverside, by the trailer park); however I also have tried the metal sinks that come in the metal counters that are in the same room, however that one eventually broke after picking up so many times.
edit: additional note: after it rains I can get wash/fill/drink options to appear and if I use the fill option, it will fill my water bottle from the parking lot below. This leads me to think it could be an issue with how the walls go transparent, but I knocked out all the windows and replaced them with full walls and the wall was not restored to a solid state.