Unable to join surface with minimal deviation

I’m wondering why I’m unable to join these surfaces, the curve deviation is <e-10
surface.3dm (407.4 KB)

is there any method other than joinEdge?

Your file has no units…

I was able to join the two parts of the left half.

I deleted the right half and mirrored the left side. The left side and its mirrored copy joined into one polysurface.

The inner edge has a few coincident control points which might cause problems in following operations.
I’d rebuild it by extracting a quarter of it, align the last two control points on both ends, mirror the curve along Y and X, then join. The result has a better control point structure.

surface.3dm (699.5 KB)

Thanks, worked following your process, any idea why the left side joins but not the right?
the file is an export from a larger model, but essentially I created the left side from mirroring the right side.

Not sure. The selected polysurface has a surface which is coplanar with the base. Maybe this has caused issues. After exploding, everything joins nicely here. Have you been changing tolerance or units?