When I click on the adjust tone curve icon (outlined circle with small arrows above and below), the curser does not change to the adjustment tool. It stays on the zoom tool. If I restart Lightroom, it works once, but then when I change photos, the problem reoccurs (until I restart again).
I came looking for an answer to the same issue. I also get this with the parallel tool in HSL, I found if I toggle between H S and L I can usually make it work.
Looks like a bug. I'm surprised no-one at Adobe has replied since you posted a week ago
Adobe has acknowledged this bug and said it will be fixed in the next release. The only reliable workaround is to roll back to version 2015.10 / 6.10. See the bug report in the official Adobe feedback forum for details: Lightroom CC 2015.10.1: Develop module: Target Adjustment Tools not working | Photoshop Family Customer Community . Please add your me-too vote and details about your issue to the bug report -- you'll be notified when the bug's status changes.
Adobe product developers don't actively participate in this forum, which is primarily user-to-user. They do actively monitor the official feedback forum and sometimes respond.