I my case under Ubuntu Studio everything works out of the box. When I boot the server, jack starts automatically.
What’s written in your post window while booting server?
Probably you did, but be sure choosing the right Interface in QjackCtl. On most Computers you can choose between Headhones and HDMI in my case called “hw:PCH,0 CX20724, Analog” for Phones and “hw:PCH HDA Intel PCH” for HDMI. If you have a discreet GPU it probably will show NVDIA or similar…