Why can't I left click on my Macbook Pro? I have a Macbook Pro (Retina, Late 2012) running OS X El Capitan (10.11.13).

What works:


Secondary click with two fingers
Feel/sound of trackpad click (no noticeable change in the feel of the trackpad)
Three/four-finger swipes/gestures


What doesn’t work:


Primary (i.e. left) click
One finger tapping to click


What I’ve tried:


Toggling System Preferences Trackpad settings  
Rebooting  
Toggling Bluetooth even though it’s normally off and I don’t own a bluetooth mouse (Can&#39;t click with trackpad or mouse (OS X 10.9, MacBook Pro))
Using a USB mechanical mouse (right click worked, left click didn’t)
Switching the Primary mouse button with the USB mechanical mouse connected (Secondary click on the left button worked, Primary click on the right button didn’t)
Reset SMC and PRAM (http://www.makeuseof.com/tag/reset-macs-smc-pram/)
Apple Hardware Test (No results found, could not click “Perform extended testing”)
Booting in Safe Mode
Checking Battery Health ("Condition: Normal” according to System Report; some people have had trouble with their battery bulging)
Laying my Macbook Pro on its side and tapping underneath the trackpad area (https://discussions.apple.com/thread/3097419?start=15&amp;tstart=0)


I’m confused.  

I’ve used the Accessibility Options Shortcut (Fn-⌘-⌥-F5) to turn on mouse keys, which I’ve been using for the past week. As fun as it is to toggle mouse keys on and off, I’d really like to get the clicking working. Please help. I will be happy if this has a stupid easy answer.
Let's just say it's a story of unknown Bluetooth interference or a paired Bluetooth device that’s in range of the Mac. 

I had this very issue last week on my Early 2013 15' MacBook Pro with Retina Display. Normal (Left) mouse-click wouldn't work under any conditions while running OS X, the built-in Trackpad, Magic Mouse or USB Mouse, while the secondary-click was fine on all.

I booted my MBP with a Linux Mint Live USB Flash Drive and the built-in Trackpad, Magic Mouse and USB Mouse all worked, both normal-click and secondary-click.  So at least I knew the built-in Trackpad, Magic Mouse and USB Mouse hardware was fine and it was looking like a software issue.

As another trouble shooting process I made an OS X USB Flash Drive Installer, figuring it would boot clean and the built-in Trackpad or Magic Mouse would work fine since it worked under Linux, however during the install... no normal-click would work, while the secondary-click was fine.

Turns out my Magic Trackpad, which I was not using and was in the closet, was depressed by a book that fell over on to it.  Because it was connected, even though not in use, it was causing the other clickable devices to fail on the normal-click while the secondary-click still worked.

This drove me crazy for a couple of hours because the book fell when I was not in the room, so I never heard it or suspected the Magic Trackpad to be an issue.  Linux Mint Live USB Flash Drive worked because the Magic Trackpad was not paired to it and therefore couldn't and didn't interfere.  Unlike in OS X it automatically connected.

What a pain it was but who doesn't love a good mystery!

Do you by chance have a Magic Trackpad lying around? 

——

Or a Bluetooth mouse in a travel bag - if that mouse is pressed down, you can’t ever get a left mouse up event from any other device.