This is really annoying. But, after trying countless ways, I finally found one that resolved the macOS Sierra installation problem 100% (and without the need to create a bootable disk via USB).
The author suggests three alternatives - I used the third and it worked on the first try. Follow the instructions carefully and you will probably be successful too. The author generously created a very easy way to follow the instructions even if you don't have a 2nd Mac.
Here is the link to the original article. To make things easier, I'll reproduce the process that worked for me. I hope it works out for you too!
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At the main macOS Sierra menu, click on the Menu Bar and select the Terminal.app
Run this command in Terminal
curl -L https://archive.org/download/sierraurl/sierra.txt
This will give you a full list of instructions and commands on your screen. You can copy and paste directly into Terminal to avoid mistakes.
Change the directory to Macintosh HD
cd /Volumes/Macintosh\ HD
Download the macOS Sierra 10.12.6 InstallOS.dmg from Apple.com using curl. Note this is a 4.7GB file and might take a while or fail. If it fails run it again.
curl http://updates-http.cdn-apple.com/2019/cert/061-39476-20191023-48f365f4-0015-4c41-9f44-39d3d2aca067/InstallOS.dmg -o InstallOS.dmg
We need to mount the InstallOS.dmg
hdiutil attach InstallOS.dmg
Now we need to install the “Install macOS Sierra.app” from the InstallOS.dmg to Macintosh HD volume
installer -pkg /Volumes/Install\ macOS/InstallOS.pkg -target /Volumes/Macintosh\ HD
The Install macOS Sierra.app is now in the /Volumes/Macintosh\ HD/Applications folder. We can now start the Sierra installer
/Volumes/Macintosh\ HD/Applications/Install\ macOS\ Sierra.app/Contents/MacOS/Installassistant_springboard
In a few seconds the Sierra installer will start BEHIND the terminal window.
If you don’t see it make the terminal window but DON’T close it as it will stop the installer.
Done. Now step through the installer you are good to go now!
(Author: Mr.Macintosh)
