You can only access macOS Upgrades via this article.
How to get old versions of macOS – Apple Support
Which gives you links to the various OS's and redirects you to
the App Store. Stupid I know.
The installer app gets downloaded to your Applications folder.
Is your mac running OK at the moment.
You may want to boot to your Recovery HD and run
First Aid/ Verify on your Disk/ Macintosh HD.
If you want to install macOS Catalina, I would recommend upgrading to macOS High Sierra first.
This should install the firmware necessary to convert the HFS+ fie system to the newer APFS file system
that newer OS's run on.
There have been reports of those trying to make direct upgrades from older Mac OS X versions
that the installations get stuck, Therefore upgrading to macOS High Sierra would be wise.
So open Safari and click on the link above.
Go to Download OS and click on macOS High Sierra.
The App Store should open and it should say Get or 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 ******.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 offering 64bit updates.
