I am running Sierra, and was working on this for a while (trying all recommended solutions). I became confounded so eventually tried restarting my computer! It worked

my conclusion is that sometimes a hard reset is necessary

I am running macOS v10.12 (Sierra) and was working on this for a while (trying all recommended solutions). I became confounded, so I eventually tried restarting my computer! It worked.

My conclusion is that sometimes a hard reset is necessary.