Thanks for the detailed help, but the most frequently occuring issue here is a discrepancy in the system date, which is likely due to a user resetting the VRam or SMC.
To enable the update server to accept the request, a user must change the date in Terminal to the current date.
Step 1) Press Power Button and immediately press ‘Command + R’ (this will boot computer into Restore Mode)
Step 2) At the option screen, there are 4 Options, however, use the mouse on the Menu Bar (top of the screen) to open ‘Terminal Utility’
Step 3) In the terminal window type the command ‘date’ which will display the time that the system thinks is the current date. If this date is not correct, (will likely say something in 2001), you’ve found the issue.
Step 4) Change the date to the correct time and date by using the ‘date’ command:
date MMDDhhmmYY
Example for January, 1st, 2018 at 12:34 am:
date 0101123418
Once you type the current date and time exactly in the format above, your system will modify the time. You can type ‘date’ then hit enter to have the system display the time, if the system now states the time as that which you entered, you’re ready to move on to restore.
You can now exit Terminal by Command + Q or selecting with the mouse from the menu File > Quit Terminal.
It will bring you back to the main screen which includes those four options. Select ‘Restore MacOS’ then hit Continue.
As long as an internet connection is active, your system will begin downloading the installer files. If you get to the Disclaimer Accept prompt, you’re connected.
PRO TIP: You should use an Ethernet cable to connect your computer to the modem (fastest) or router (faster, depending on hardware), but you can also use Wi-fi (slowest).
You can use Terminal Utility to determine whether you have an internet connection (I’d leave more details here but you should find a help article for that elsewhere.
