This might be a package configuration problem. I've installed Qtractor on Plasma and dragged an mp3 file to the file list.
When I press Space, it attempts to start the track but I get an error
the audio/MIDI engine could not be started
Make sure the JACK/Pipewire audio service and the ALSA Sequencer kernel module (snd-seq-midi) are up and running and then restart the session
Make sure there aren't other things also set to use Space as a keyboard shortcut
System Settings - search for shortcuts
In the shortcuts, search for space
Do you have any other shortcuts set to just use Space?