MacBook Pro 15" Retina Mid 2015 Trackpad Not Clicking
I went in to the Genius Bar and got an answer, so here it is!

Note that this may not work for everyone, but it did work for me.


The first thing the tech did was have me save everything and power off the computer. Wait until the screen is completely off and everything is powered down. 
Plug in your MacBook. 
Press the following keys in the bottom left of the keyboard: ctrl+option+shift (holding them all at the same time) and then tap the power button key in the very top right.
If the light on the charger is amber, it should flash green and then go back to amber. If it was already green, it will stay green.


This is called an SMC reset, or system management controller. 

The way the tech explained it to me, this resets how the computer distributes power, and should make the computer send the correct amount of power back to the trackpad. I don't know if that's exactly how it works, but that's basically how he explained it to me.

After doing this SMC reset, the trackpad started working immediately. Hopefully this works for you as well!