As many people have found, the default jackd version that comes with the Pine64 Ubuntu image (which is version 1.9.10) will crash as soon as you start it with a bus error and a message in the kernel log about an aligment problem.
In later versions of jackd this has been solved, but as there is no package, you're currently required to compile it by hand. This is not as difficult as it might sound, but it requires some tinkering in the terminal. Also: make sure that your pine64 board has enough power. If you use a 2.0A rated power supply, the board might crash in the middle of the process (as has happened to me). This tutorial assumes you are working with either the Ubuntu Basic Image, or the Ubuntu Mate image. If you are running a different image, the same process will apply, but you might need to adjust some actions.