What OS do you want to install, there is a world of a difference between El Capitan and Catalina.
When you tried to install El Capitan what happened, from the link I gave you at Section 4
you would click Download OS X El Capitan, this downloads Install MacOSX.dmg, open that to
get Install.MacOSX.pkg, double-click on that and an installation window will open, this does not
install El Capitan but converts the .pkg to the Install OS X El Capitan.app that appears in your
Applications folder, clicking on that will start the installation of El Capitan, or you can use it to
create a bootable USB, How to create a bootable installer for macOS – Apple Support
Do you really want to jump straight up to Catalina, you can do it but be prepared for big changes.
just use the Install macOS Catalina.app or you can make a bootable installer as suggested above.
As you have already tried the Catalina install app be prepared that it does look like it is doing
nothing for a long time but be patient. The main Install macOS Catalina screen will sit for about
10 minutes, perhaps more before it restarts to a back screen with the white Apple logo and
a white progress bar that will give you a very loose estimate of the time it will take.
Remember the Catalina install will be converting your disk from HFS+ to APFS.
Personally I would go to Mojave first, again your disk will be converted to APFS
but Mojave is a more kinda warm sand between the toes feeling rather than the
cold unrelenting waves of uncertainty that surround Catalina.
Whatever you decide do your homework first and make those back ups.