Reinstall OS X El Captain

Doesn't really answer my question.

What method are you using to get to this stage.

Are you Restarting the mac while pressing and holding down the Command and R keys,

this restarts the mac to its Recovery HD.

Or are you Restarting your mac while pressing and holding the option/ alt key

and selecting a bootable USB flash drive installer.from the Startup Manager.

Does your mac have an operating system on it at the moment if so what OS is it

and is it working OK.

You could try different dates instead of 2017, try 2016 and 2018.

Do you know what the year of this mac is, does it have to be OS X El Capitan that gets reinstalled

If you can't get reinstalled from the Apple Recovery Servers then you could make a bootable USB flash drive

installer for El Capitan, but you can only do this with a mac that can actually run OS X El Capitan.

Any newer mac that came preinstalled with a newer OS than El Capitan cannot do this procedure.

How to get old versions of macOS – Apple Support

Go to Download OS and click on OS X El Capitan 10.11

this downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan.

This includes macs that have the potential to run El Capitan but have been upgraded to a newer OS.

A mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

(If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

Create a bootable USB installer disk using the Install OS X El Capitan.app in the Applications 

folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

The certificates for several of Apples OS's expired in October 2019, Apple haven’t bothered 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 011421002017

press Return

enter your password

press Return

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

just enter 

date -u 011421002017

press Return

You won't be prompted for a Password if you did not need to use sudo

Once the date has changed you can quit Terminal.

Now try downloading the OS.

Click on Install OS X, press Continue.

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.

Are you trying to reinstall from Apples Recovery Servers or are you using a bootable USB installer.

What method are you using to install OS X El Capitan.

Is your mac functioning and does it have an OS on it that works.

Are you following the instructions.

Is the mac connected to the internet via cable and not WiFi.

Make sure WiFi is switched off.

Did you change the date as described using Terminal from Utilities in the menubar.

Was the date change accepted.

Did you Quit Terminal

When you click on Reinstall OS X what happens next.

Do you have another mac in the house or one you can borrow.

It needs to be able to run El Capitan.

Mac from 2009 can only run up to OS X El Capitan unless it is a late 2009 iMac.

Are you able to borrow a mac you know is capable of running El Capitan

so you can make the bootable USB installer and use that to install El Capitan

on the first mac.

So if you are Restarting while pressing the Command and R keys then you are restarting your mac to the Recovery HD,

try the instructions I gave you earlier.