What finally resolved it was to delete the half-cooked “Install mac OS Sierra” file from Launchpad (which had a big question mark on it), then rebooting.
If the Mac App Stores shows that “mac OS Sierra” has “Downloaded” and the button is not possible to click again, you likely had one of the beta or GM builds and you will need to remove any existing “Install mac OS Sierra” application installers from the Mac, or any connected drives.
Some Mac OS Sierra users have reported similar issues with their Mac getting stuck on a black screen when waking from sleep.
Often the same SMC and NVRAM reset procedure remedies those difficulties.
We’re compiling a list of common problems in this troubleshooting guide to remedy issues with the mac OS Sierra download, installation, update, and post-install process.
But, not everything goes smoothly for everyone, and sometimes updating to mac OS Sierra can lead to a variety of problems.
To be perfectly clear this is meant to be a compilation for potential issues experienced with the mac OS Sierra update process.
Most of this will not be encountered by the average user, and is in no way suggestive of what to expect during the Sierra update or installation process.
Booting a Mac into safe mode is easy, just hold down the SHIFT key on system boot until you see the bootup progress bar, then let go.
Safe Mode will clear out some caches and disable some functionality, but it can often be a helpful troubleshooting measure.