Can't find out what the problem why the definition is not working

I’m working on a urban parametric design and my definition is not working… it just needs to extrude but somehow one of the boundary surface node is not working as picture shows. Can you guys help me out with it please?

시험용.3dm (132.7 KB)
시험용.gh (14.3 KB)

Set a plane in the last input of RDiff and make sure A and B input values are coplanar within a very low desviation. Also make sure curves are closed.

시험용 (9.8 KB)

Sorry, I’m new to grasshopper, I only understood the first part, but the second part is quite confusing. Could you possibly explain it in a more sophisticated way please?

Sometimes happens that two curves looks coplanar but if you zoom in, there are not really coplanar. This can be a reason why RDiff doesn’t work. But looks like your problem was another thing.