'Green Screen Keyer' not letting me choose the keyed color

Press A on your keyboard to ensure the Select tool is chosen.

Make sure your clip in the timeline is selected.

In the keyer in the Inspector click on the square Sample Colour box.

Check that you have not got the Transform or any other tool active.

Yes, I have been caught out several times with that and other effects when I have not clicked the "Done" button in Transform.

I only remembered that after I posted my original suggestions.