Hi simba, I see that there are a lot of curve objects in your file with the curves sitting right on top of edges - that can be problematic because for N-sided to work you need to select edges, not just curves. So try hiding or deleting those curves first and that may help.

Also another thing that can cause n-sided to fail is if the edges do not touch each other at endpoints, like if there is too much of a gap between them. It looks like you have this problem in some areas of your model, like for example here there seems to be a gap of around 0.03 units in size, that's too large and it makes NSided not recognize the chain of edges as a closed loop:





- Michael