In short, the SynoCommunity packages doesn't work because they're compiled in a way that the new DS416j doesn't support (more info here). Luckily, Cambier has recompiled a handful of these packages in a way that can be read without a problem.
Installing SABnzbd on the DS416j
From your DiskStation interface, go to PackageCenter > Settings > Package Sources and add the URL
https://synology.cambier.org/ to the list and click OK.
Back in the Package Center, you should see a Community section on the left panel (you might need to hit Refresh). From the Community section, install the Python package.
Now, still in the Community section, click on SABnzb's Install button. After this completes, you should be good to go!
From this point, the setup of SABnzbd is just like in any other Synology. Good luck!