Cannot find Catalina 10.15 installer

Before you go to an operating system that strictly supports only 64-bit applications, I suggest that you run the free Go64 app to see what 32-bit applications you are using on Yosemite that are not supported on Catalina. This would hold true for printer/scanner drivers too.

Then, using the Safari browser only, visit How to get old versions of macOS where you will find the link to the full Catalina installer in the Mac App Store.

It would be a good idea to get a last Time Machine backup before the upgrade, and then do not use that backup drive with Catalina.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

Tips Big Sir Upgrade Released Nov 2020

Tips Monterey Upgrade  Released Oct 25, 2021

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

The page How to get old versions of macOS contains links to download older versions of MacOS.

This is the same page you referenced in your post. Please note that you must use Safari to download Catalina and other old versions of MacOS. The links do not work with Firefox and other browsers.

In case it helps, I always use the links on the below page "How to create a bootable installer for macOS" to download installers.

https://support.apple.com/en-us/HT201372