The displacement modifier on your Plane object is displacing the plane geometry. The FLIP Fluids surface mesh is contained in the fluid_surface object. So to get those waves to appear on the simulation, you would need to add the displacement modifier to the fluid_surface object to displace the fluid geometry.
One way to duplicate the modifier from your plane to the fluid_surface is to use the CTRL+L ‘Make Links’ menu. Documentation: https://docs.blender.org/manual/en/latest/scene_layout/object/editing/duplication.html#make-link
I have a post here that might be helpful: FLIP Fluids Addon: A liquid fluid simulation tool for Blender
This post describes a similar technique to add displacement to the fluid_surface and contains a sample .blend file. The only difference is that it uses an Ocean Modifier instead of a Displacement Modifer, but the technique is similar.
Hope this helps!