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.

Chroma Key with video

Chroma key with image

You can check out our Youtube video tutorial showing how to use the Chroma Key here:

https://www.youtube.com/playlist?list=PLmqmCnrRTizrGszCdjHH357X_QSthg0xU