I tried installing mixxx from terminal, and succeeded. Here's the APT history:
Start-Date: 2022-01-04 11:54:45
Commandline: apt install mixxx
Requested-By: user (1000)
Install: libebur128-1:amd64 (1.2.4-2, automatic), mixxx:amd64 (2.2.0~dfsg-1+rb2), mixxx-data:amd64 (2.2.0~dfsg-1+rb2, automatic), libhidapi-libusb0:amd64 (0.8.0~rc1+git20140818.d17db57+dfsg-2, automatic), libopusfile0:amd64 (0.9+20170913-1, automatic), libqt5scripttools5:amd64 (5.15.1.2+dde-1+eagle, automatic)
End-Date: 2022-01-04 11:54:54
Hope it helps.


中文 
Hey Team and all,
Just installed Deepin (my first linux OS for about 15 years) and lovin it so far, great to see how far things have jumped. It's almost as new-user friendly as Windoze, and more fun to fettle with!
2 issues I've hit so far and been unable to fathom - hoping you lovely folk could help...
Mixxx is on the app store but won't install - "Lack of dependency, will solve soon" - tried to manually install, getting nowhere.
Spotify wasn't on the app store, so I followed this guide to install via Flatpak;
https://tutorialforlinux.com/2020/09/25/step-by-step-spotify-deepin-linux-installation-guide/
Installed Flatpak no worries, installed Spotify fine too (once I remembered to run from Terminal as SU) but it won't run. Rebooted - same.
Tried it's alternative Spot - same again; installed but won't run.
Also tried the other potential; Nuclear, installed but won't run - definite pattern here ;) (Rebooted after each install, just in case)
Installed MIXXX from Flatpak too, again I get an icon that wiggles on click but does nothing.
Any suggestions or solutions from you lovely lot?
Cheers all,
Jay.