Hello, the cabinet needs to be against the wall/column or have an obstruction to generate the back water barrier by default. If there is a gap, there will be no back water barrier. You can handle it as follows:

Method 1: Right-click the countertop to delete it, then drag the cabinet again to make it automatically attach to the wall/column, and then right-click to generate the countertop;

Method 2: If you don't want to move the cabinet, you can right-click the countertop - select Countertop Edit, modify the left and right extension values so that the countertop is against the wall (Note: The extension value should be larger than the gap to be generated. For example, if the gap is 50, you can change it to 60 or 70)