Before we try to rule out wrong MIDI connections or a missing
soundfont, we should start with a command line to start jackd.
Run
aplay -l
in a terminal and post the output.
It will show "card 0", "card 1" ... "card n".
Then in a terminal test running
card=0; killall -9 jackd && jackd -dalsa -dhw:$card -r48000 -p1024 -n2
after that start QjackCtl, Qsynth and Qtractor and at last connect the
MIDI and audio IOs.
If it shouldn't work close Qtractor, Qsynth and QjackCtl and run
card=1; killall -9 jackd && jackd -dalsa -dhw:$card -r48000 -p1024 -n2
after that start QjackCtl, Qsynth and Qtractor and at last connect the
MIDI and audio IOs.
If it shouldn't work close QjackCtl and continue ...
If it never should work and there should be failure messages, then post
those messages. If it shouldn't work, but there should be no failure
messages then report back and we'll focus on MIDI connections and
soundfonts.