How I can add a Face Retouch Effect to a FaceCrop texture?

If you select the Face Crop Texture in the Resources panel, there are some settings for it in the Inspector panel. By default it is grabbing the face from the Device Camera Texture. What you can do is add a Screen Texture in the Resources panel and use that as the input to the Face Crop Texture. Then just add your face retouch to the original camera normally.

The screen texture is kind of like a render target, but you don't need to change any camera settings. Keep in mind that the screen texture may grab other elements of your scene, not just the face so you may need to reorder things. If that isn't working, you can create a new camera/layer/render target combo, put the face retouch on that, and then feed that render target into the face crop texture.

The exact setup will depend on how your scene is setup, but it comes down to changing the input texture on the face crop texture.

If you do go with the separate render targets, make sure the face retouch is on the same layer as the camera outputting to the new render target.