Mouse clicks not working on MacBook Pro, with both track pad and USB mouse
Here is what worked for me.

I am on a MBP Retina 13" Late 2013 and macOS Sierra. The pointer started to have erratic movements and generated automatic clicks. Then this stopped but the left click would not work at all, even with an external mouse (only the right click of the mouse worked).

I tried everything (NVRAM reset,...) but nothing worked.

Then I found a post (here) linking it to an issue with an external monitor - I plugged the HDMi of the external screen I use from time to time, turned the monitor on and voila !, the left click worked again !

The post is 3 years old but he bug seems to be surviving...