MacOS will not reinstall after Factory Reset

The certificates for several of Apples OS's expired in October 2019, Apple have been very poor at getting these updated on their Recovery Servers with valid certificates. This may be the problem you are experiencing. 

Try this workaround, we are going to set the time and date on your mac to a date previous to the expiry date of the certificates.

Connect your mac to your router via cable, not WiFi

Boot to your Recovery HD, click on Utilities in the menubar select Terminal.

Make sure WiFi is switched off, it can reset the date back to today.

Enter a new date, for example or just copy and paste

sudo date -u 011421002016

press Return

enter your password

press Return

If Terminal returns an error saying sudo : command not found, then try again without sudo.

Now try downloading the OS.

If this works then when the OS is installed and booted up you can Open System Preferences> Date & Time

and reset the time back to today.

I'm not sure, exactly. A lucky hit after a few shots in the dark.

I printed out the advice you gave me in response to my separate post and then I fell at the first hurdle.

Your advice began: "1. In the Disk Utility app  on your Mac, choose View > Show All Devices."

My version of Disk Utility does not have the 'View' option. Instead I just took guesses selecting whatever disks/volumes I could see on the left and then clicking the First Aid function. At least half of these attempts indicated that the process had failed. However, I decided to try another reinstall of Sierra anyway as I had nothing to lose. And, like magic, the installation worked! My iMac is fully functional.

So, thank you! (I think...)

Ignore my last message. Problem solved!

Hi Eau

Will your suggested remedy also apply to my problem outlined here:

My problem: Bought an iMac off a friend. Deleted all his files and tried to do a factory reset. When I rebooted, instead of the Apple Logo I got that scary file icon with a ? on it. So tried Command+R to go into recovery mode and opted to re-install MacOS Sierra. Every thing seems fine as the installation counts down from 6 minutes (although it takes more than 6 minutes). Then, with about 12 seconds to go, the remaining time changes to a negative amount (-399 seconds) and I get this message: "An error occurred while preparing the installation. Try running this application again." I've tried about 5 times now and the result is always the same.

My iMac specs:

Model: A1418 Z0TK0002K

21.5 inch

3.0QC processor /16GB Ram / 1TB HDD / RadeonPro555

What was the cure?

Great, take care my friend. :)