When the top/bottom lines appear black after rendering, first check if the overlap of the top/bottom lines (enter the wireframe mode by pressing Ctrl+2 for easy checking. If you see multiple lines overlapping and flashing together, it indicates an overlap) causes the blackening, and then regenerate the top/bottom lines.

The operation is as follows:

Step 1: Open the solution, adjust the view to the front of the cabinet, and click on the top/bottom lines respectively. When you find that there is an overlapping part when clicking the top/bottom lines with the left mouse button, delete the top/bottom lines first;

Step 2: Check if there is an overlap of the cabinet and re-place the cabinet;

Step 3: Right-click on the wall, select All Lines - Install Top/Bottom Lines;

Step 4: Select the style and material, and modify the section height.