You must have OptiFine or Iris Shaders installed. These mods enable the "Resource Pack" features and shader support required for high-end textures.
This gives 2D textures a 3D "depth" effect, making bricks or gravel look like they physically pop out. ULT_Texture_Pack_PT_6_(INSTALL).zip
Back in the Minecraft Resource Packs menu, move the pack from the "Available" column to the "Selected" column. You must have OptiFine or Iris Shaders installed