How to create a bootable disk for an older Mac?
Note: A more complete and up to date answer to this question can be found at this link: How can I download an older version of OS X/macOS?

I tried your link https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12 and received the following error message. Assume your error message was the same.



The answer below assumes the Mac is running High Sierra.

If you purchased El Capitan for free in the past, then you will find this recorded in your App Store account. Open the App Store application, then click on the Purchased button. Enter your username and password. If you purchased El Capitan in the passed, then El Capitan will appear in the list. You will be able to download even if the macOS is newer than El Capitan. 

If El Capitan does not appear in your list of purchases, then try to find someone who has purchased El Capitan and use their username and password.