I have written two versions depending on the exact nature of your problem.
Struggling with the SketchUp Round Corner extension? đź§© sketchup round corner crack top
Do not apply a 2-inch radius in one click. I have written two versions depending on the
SketchUp models often have "hidden geometry" or stray lines that users aren't aware of. If you try to round an edge that crosses a hidden intersection line, the plugin gets confused about where the surface stops and starts, leading to a tear in the mesh. Avoid "Sharp" Offset on Tops Do not apply
This gives you total control and eliminates the crack because the top face never touched the rounding algorithm.
SketchUp’s geometry engine has a hard time calculating intersections when curves meet at complex corners. RoundCorner works by projecting arcs along edges. When three or more arcs meet at a corner, the plugin has to calculate a complex surface to close that corner.