That is correct. There are a number of effects and transitions that are not available in the Mac version of the program. In my book, available on Amazon, I include a complete list.
Blue Screen Key, Green Screen Key and Chroma Key are among those effects.
However, the Videomerge effect should serve as an effective alternative. In fact, I often prefer using it over the Key effects.
This is not Adobe. So, I cannot give you the why. But, typically Premiere Elements Mac keying effects do not include Chroma Key, Green Screen Key, Blue Screen Key.
Many suggest the use of Videomerge to the Premiere Elements Mac user for green screen work. I do not. I prefer the Difference Matte and have written details on its use in this regard.
Please consider, and then let us know the outcome as your schedule permits.
Best wishes for the new year.