If the copy that you downloaded of El Capitan is older than late-2019, see also this article:
https://tidbits.com/2019/10/28/redow...-certificates/ for why you may need to re-download the installer.
I think that may be why the recovery partition is not working to install the OS: that the recovery partition was created before the signing certificates expired in October 2019. That happened to me with an iMac I was trading in around that time, and making a bootable installer allowed me to re-install the OS before I shipped the system back.