Chroma Key feature can only be applied to video and image layers. It's not possible to apply Chroma Key to videos and images on the primary timeline.

Screentshot : Chroma Key Videos (Chroma Key with video)
Screenshot : Chroma Key Images (Chroma key with image)

You can check out our Youtube video tutorial showing how to use the Chroma Key here:
https://www.youtube.com/watch?v=x_K2vblywp0