always same error...

This error message is, I think, self-explanatory :).

To run Qtractor you must have a current sane JACK (Jack Audio Connection Kit) service running smooth against your preferred sound-card device AND you must have ALSA sequencer kernel module loaded (snd-seq-midi; usually you should instruct this at default system init configuration).

On every Linux system and distro, there's always one configuration step that you MUST do before ever starting jackd (the proper JACK server). That means giving real-time scheduling privileges to your login user or group. Otherwise, you can be sure that jackd won't start to run at all. Please, look after your /etc/security/limits.conf file, if you haven't already). It must have some line of the sort:

assuming your user belongs to the audio group. You can replace audio by any other group name you find appropriate. The number shown, 90, is also arbitrary, you'll be fine if you find any number like this, valued between 90 and 99.

There could be many other issues that stops JACK to work on fresh installed system, but once you clear that early obstacles, Qtractor will just start alright ;)

Cheers