When Apple releases an OS the first version is called 10.15 in the case of Catalina.
When Apple updates that version then it becomes 10.15.1, Apple then removes the
previous version 10.15 from its servers, the process repeats until what is the latest
version of Catalina which is 10.15.7 and is available via the App Store.
Its sounds like you were trying to install updates for Catalina, these updates from the
Apple Support Downloads site require you to be running an earlier version of Catalina
before you can install them.
Open your Safari browser and click on the link below,
How to get old versions of macOS – Apple Support
Go to Download OS and click on macOS Catalina
this will redirect you to the App Store Catalina download page
it should say Get or Download.
This will download the full installer app for Catalina which will download
directly to your Applications folder as Install macOS Catalina.app this is all
you need to install Catalina. It installs the last/ latest version 10.15.7,
it is 8.26GBs in size so takes a while to download.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS Catalina.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you are considering upgrading to Catalina be warned it does not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are providing 64bit updates or alternatives.