• Error: The above package list contains packages which cannot be
• installed at the same time on the same system.

(media-sound/jack-audio-connection-kit-0.125.0:0/0::media-kit, installed) pulled in by
>=media-sound/jack-audio-connection-kit-0.121.3-r1[-mgorny(-)] required by (virtual/jack-0:0/0::media-kit, installed) USE=""

(media-sound/jack2-1.9.17:2/2::media-kit, ebuild scheduled for merge) pulled in by
media-sound/jack2[-mgorny(-)] required by (media-plugins/gst-plugins-jack-1.18.4-r1:1.0/1.0::media-kit, ebuild scheduled for merge) USE="-test"

also

equery d virtual/jack

• These packages depend on virtual/jack:
  app-emulation/qemu-5.2.0-r3 (jack ? virtual/jack)
  media-libs/mlt-7.2.0 (virtual/jack)
  media-libs/openal-1.19.1 (jack ? virtual/jack[-mgorny(-)])
  media-libs/portaudio-19.06.00-r2 (jack ? virtual/jack[-mgorny(-)])
  media-plugins/alsa-plugins-1.2.2 (jack ? virtual/jack[-mgorny(-)])
  media-plugins/gst-plugins-jack-1.14.4 (virtual/jack[-mgorny(-)])
  media-sound/audacity-2.3.3 (jack ? virtual/jack)
  media-sound/fluidsynth-2.0.4 (jack ? virtual/jack[-mgorny(-)])
  media-sound/mpg123-1.25.10-r1 (jack ? virtual/jack)
  media-sound/pulseaudio-14.0 (jack ? virtual/jack)
  media-video/ffmpeg-4.3.1-r1 (jack ? virtual/jack[-mgorny(-)])
  media-video/obs-studio-27.1.3 (jack ? virtual/jack)
  media-video/simplescreenrecorder-0.3.11-r2 (jack ? virtual/jack)
  media-video/vlc-3.0.16 (jack ? virtual/jack)

thus media-plugins/gst-plugins-jack has 2 incompatible dependencies