I have a Macbook Pro from 2013, was on Yosemite then updated to El Capitan yesterday to see if it would fix things, which it didn't. Basically what the title says: I can't do anything that requires clicking and dragging, while everything else works perfectly. The odd thing is that it seems to be dependent on where I'm starting from on the screen – picking up a file on the far right of my desktop works most of the time, but it always cuts out after a second or less. It never works on the left half of the screen, and very rarely in the middle.
The same problem exists in guest and safe mode. I've tried deleting the com.apple.finder.plist file in library/preferences but that didn't do anything. I suspect a visit to the genius bar is in order, but just wanted to double check here first.
EDIT: it works with a graphics tablet, just not the touchpad.