When there is no backsplash for the countertop, first check if the cabinet has a distance from the wall and modify the distance to 0.

Step 1: Right-click on the countertop, select Delete, left-click on the cabinet, check if there is a distance from the wall for the cabinet, and modify the size to 0;

Step 2: Right-click on the wall, select Countertop - Countertop Installation;

Step 3: Select the material and click OK.

Operation completed: