I would suggest starting jackd from the command line, and posting the
entire set of messages starting with the command line you typed to the end
of the startup messages.

Perhaps there is a permission problem with your user account which is not
allowing shared memory setup.

[moshew@moshe-laptop ~]$ jackd --realtime --realtime-priority 89 -dalsa -dhw:D1248 -r48000 --period 128 -n2
jackdmp 1.9.12
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2016 Grame.
Copyright 2016-2017 Filipe Coelho.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
no message buffer overruns
no message buffer overruns
no message buffer overruns
JACK server starting in realtime mode with priority 89
self-connect-mode is "Don't restrict self connect requests"
audio_reservation_init
Acquire audio card Audio2
creating alsa driver ... hw:D1248|hw:D1248|128|2|48000|0|0|nomon|swmeter|-|32bit
configuring for 48000Hz, period = 128 frames (2.7 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 24bit little-endian in 3bytes format
ALSA: use 2 periods for capture
ALSA: final selected sample format for playback: 24bit little-endian in 3bytes format
ALSA: use 2 periods for playback