ve had an issue with the white balance picker tool for years! Every so often when I choose it, and go to use it, it gives me the zoom tool and no matter what I do, I cant pick the white picker tool, it gives me the zoom tool everytime!
This tends to start happening when I import a large number of high res pics, say 400 - 500 and the first 30-50 or so grade fine (I build a present, then copy it to images and tweak accordingly). Once it happens, I have to restart Lightroom and then can sometimes do anything between 10 - 50 images again before the issue reoccurs.
Its been listed on this forum previously and someone posted with a similar issue just recently. For reference, this was in Lightroom CC 5.4 and just upgrade to 5.5 and has occured for the last few years repeatedly and every upgrade, I hope its a bug thats fixed but always returns. I even reformatted this PC last year hoping to fix it!
Below is a video of the issue happening. As you can see, the white balance picker works fine but at about 18s, you no longer can click it and the tool is stuck in the zoom mode. (FYI, for some reason the screen record has a slight mouse offset but safe to say I was definitely clicking on the icon!)