Really Strange Behaviour on Trackpad/Keyboard
This happens all the time for various reasons. Typically, you want to disable Bluetooth and disconnect all usb devices. Then verify that you only pair or connect one device at a a time. 


  if one device has left mouse pressed down, you can’t ever get a left mouse up event from any other device.


Bluetooth interference or less commonly, faulty usb devices can overload the communications channel and you can isolate this by connecting devices one by one. 

Why can&#39;t I left click on my Macbook Pro?

The last two times I had this affect me was a second Magic Trackpad pinned under a book on a bookshelf and an old but reliable (up to when it failed) USB 2.0 hub.