after giving an update, the system still asking for same up

Hi ehsan1000,

We see you are needing upgrade to a newer version of your operating system. We can help with this. You will need to update to El Capitan 10.11.6 first, then the option to update to Catalina will be available.

How to update the software on your Mac

Specifically we want to check out this section.

"If you're using an earlier macOS, such as macOS High Sierra, Sierra, El Capitan, or earlier,* follow these steps to keep it up to date:

1. Open the App Store app on your Mac.
2. Click Updates in the App Store toolbar.
3. Use the Update buttons to download and install any updates listed.
4. When the App Store shows no more updates, the installed version of macOS and all of its apps are up to date. That includes Safari, iTunes, iBooks, Messages, Mail, Calendar, Photos, and FaceTime. Later versions may be available by upgrading your macOS.

To automatically download updates in the future, choose Apple menu  > System Preferences, click App Store, then select ”Download newly available updates in the background.” Your Mac will notify you when updates are ready to install."

How to upgrade to macOS Catalina

Have a great week!

The App Store Updates page only informs you of updates available for the OS you are currently running.

As you are running an older system then you are probably up to date on that system.

If you want to install a newer OS that is called an Upgrade. 

The model and year of your Mac will determine which macOS 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

How to upgrade to OS X El Capitan – 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