Hey there and welcome!
I looked into this and was able to reproduce the issue with a fresh download package after clearing my machines NuGet cache. However, running the current source build, I wasn't able to reproduce it again even though I cleared the NuGet cache after every try. Going back to the binary download installer, the issue turned out to be a bit erratic, sometimes showing for all the packages, sometimes only for some, and sometimes not at all.
My best guess so far is that the NuGet API endpoint is temporarily experiencing problems of some sort, with the exact way in which packages are queried potentially influencing success rate or reproducability of the issues.
I've rebuilt the installer package from the latest source version and replaced the one that is up for download. There shouldn't have been any big changes in the mean time, but it's possible that NuGet query behavior has changed - the last installer was from December 2016. Can you re-download the installer and try again?
(Please clear your browser cache before you do so we can rule out any interference from that side, though I suppose the browser should be clever enough for this.)
In any case, since this might be a network-related issue, it doesn't hurt to just give the installer a few tries in case there really are connectivity problems. Due to both the machine-wide and the Duality-local NuGet package cache, it won't need to re-download a package once it's been there, so even if you only get a subset of packages on every try, you'll eventually end up with a full set unless there's a full breakdown.
Anyway, let me know if the new installer fixes this Edit:
It seems like there is indeed some slight instability in the NuGet V2 lookup / search
both yesterday and today. See Pingdom history on status.nuget.org