Update Operating System

In addition to the above...

If a Catalina update is failing, please post the messages or diagnostics shown, if any.

Try updating Catalina while booted in Safe Mode, too. That disables various add-ons.

If the Safe Mode test fails, download EtreCheck and run it, and open a new reply here and then press the button that looks like a printed page to get a text input box big enough to paste the hardware and software configuration report here. The report can show the configuration, and can help identify potential problems.

Are you really running Catalina, if so that is currently the latest macOS, so there is no upgrade.

Your profile says 10.11 which is El Capitan, Catalina is 10.15.

If you are on El Capitan and want to upgrade to a newer OS then the model and year of your mac

will determine which OS you can upgrade to.

Please use Safari to access the following links do not use Firefox

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

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 that 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/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support

What happens when you try?