Knockouts need:

1.    a layer to define the knock-out shape

2.    the layer to be knocked through, and

3.    a background layer.

Make the top layer a clipping mask:

Layer/

On the layer to be cut through, deselect blend clipped layers as a group

Go back to the clipping layer. change fill to 0% and knockout to deep [to go to the background]

You can also use blend modes and other options:

If you want to restrict blending to an individual layer [in this case, to dim the clouds], turn off Blend clipped layers as Group to the cloud layer. [If you don't, both the clouds and the sun get dim.]