Been a while since I tried to upload anything to the gallery.
I seem to remember at one point there being issues concerning the version of NuGet being used.
The last installed version I have reports itself as 2.6, although I can't recall if I reverted back to an older version.
I only ever used NuGet to upload to the gallery, which was so infrequent that I had to reacquaint myself each time and invariably got it wrong, so all I can suggest (and with some reservation) is what worked then, try a different version of NuGet,
fiddle with the settings and persevere.
Sorry I can't be of more help.