Make sure the modifier is part of the model you are trying to modify and is not associated with another model.
See how the Cube (red arrows) has the Sphere modifier in it but the modifier is actually part of the Cylinder part? IE the Sphere is listed under the Cylinder (with the little green plus symbol) If you slice this the cube won’t have any modified infill.
You need to select and then Right click / “Merge” the parts on the plate together and then it will work.
