I consider myself new to Houdini. There was a problem with the paint mask in Heightfield. I have attached a video screenshot. The mask is not displayed on the surface even though everything is connected. Please help me break it down. Thank you in advance.

I'm guessing the "mask" heightfield isn't actually there, maybe because it was deleted before the heightfield was cached to disk. First re-create the mask layer before trying to paint it. You can use Heightfield Copy Layer to do that. Make sure to hit Reset Strokes on Heightfield Paint before attempting to paint again.