Rhino 6 Mesh

I’m new in working with meshes in rhino and i failed to “cut” a hole in a terrain mesh i made.
Without mesh i would boolean the polyline/surface from my terrain but i failed doing it with my mesh.
I tried to use the boolean mesh tools and also the split curve from mesh but it didn’t worked - nothing happend.

As I said wirhout meshes i would know what to do - is there any mesh tool i can use for this?
I hope my question is understandable.
Thanks and greetings

Hi - intersection and trimming operations with meshes in Rhino 6 is rather limited and the tools are in the process of being rewritten for Rhino 7. You could try the WIP of Rhino 7 and see if your issue is resolved there or you could post a 3dm file so that someone can take a more detailed look.