Pronterface won't connect to my Anycubic Kossel Linear Plus
I know this is coming very late, but I'll post anyway in hopes that it may help somebody else experiencing the same frustration.

I received all of the same suggestions again and again on the forums -- about baud rate, COM ports, and a lot else which failed to get Pronterface to connect.

At last I posted to the Anycubic User Group on FB, and quickly received what was in my case the solution -- and so ridiculously simple.

I was told to hit RESET two or three times on Pronterface after hitting CONNECT.  Yup, that did it!  

Good luck!!  :D