How to download El Capitan (for MacBook5,1) from a MacBook Pro (running macOS Mojave)

First question have you ever downloaded El Capitan before. If you have login to the App Store (not on the MBP with Mojave)

login with the AppleID you used before and click on Purchased, if you see it on the list, click download.

Second question the MacBook you are trying to upgrade to El Capitan, what OS is it running at the moment,

if it is on MacOS X Snow Leopard 10.6.8, Lion, Mavericks or Yosemite use the link above to download El Capitan. When the download completes, and before the installation starts, open your Applications folder and copy Install Mac OS X El Capitan.app to a external disk or USB stick. you can then make a bootable USB with it, you can then boot from that USB, open Utilities from the menubar, select Disk Utility, Erase the hard drive and then perform a clean install of El Capitan.

You are getting the error on the MBP because it is running Mojave which is a newer OS from the one you are trying to download, the App Store won't let you do that.

Another option is to use another Mac if you have one or can borrow one that cannot update further than El Capitan,

use the link you posted and the procedure I described to get the Install Mac OS X El Capitan.app on to USB.

You can make a bootable USB stick to install using these free programs which will do all the work for you.

Bootable USB Flash Drive – Diskmaker X      Disk Creator   or use Apple’s method       Create a bootable installer for OS