Error restoring macos El Capitán

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.

Before you made the mistake of resetting your mac which operating system was it running.

If your mac was running macOS High Sierra, Mojave, Catalina or Big Sur then the drive would

have been formatted as APFS.

Mac OS X El Capitan can only be installed on a disk formatted as Mac OS Extended (Journaled).

Boot to your Recovery HD open Disk Utility click on View in the menubar and

select Show All Devices.

Highlight the Disk probably called Apple HDD/SSD .... or similar.

Click Erase.

Give the Disk a name.

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Map.

Click Erase.

When Done quit Disk Utility.

Now try the procedure in my first post.

So it was running Mac OS X El Capitan before you reset the mac.

Slightly different instructions.

Boot to your Recovery HD open Disk Utility.

Highlight the Disk probably called Apple HDD/SSD .... or similar.

Click Erase.

Give the Disk a name.

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Map.

Click Erase.

When Done quit Disk Utility.

Now try the procedure in my first post.

If none of these things work do you have another mac you can borrow,

this mac must have the potential to run Mac OS X El Capitan.

I may be a mac that once ran El Capitan but has been upgraded to a newer OS.