Go to View > Hidden Geometry . You may find that the "crack" is actually an unsoftened edge or a missing tiny triangle. Use the Line Tool to manually stitch the gap, then use the Eraser Tool + Ctrl to soften the new edge.
Please tell me which one you are looking for so I can provide the correct content! How To Round Edges In Sketchup sketchup round corner crack top
If the crack is small and you don't want to redo the operation: RoundCorner | SketchUcation Go to View > Hidden Geometry