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.
First make sure your mac is connected to your router via cable not WiFi, if the WiFi is on turn it off.
Boot to your Recovery HD, click on Utilities in the menubar select Terminal.
Enter a new date, for example or just copy and paste
sudo date -u 011421002018
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.