You need a MacBook Pro from 2012 in order to upgrade to Catalina.
Open your Safari browser and click on this link, How to upgrade to macOS Catalina – Apple Support
Go to Section 4 and click on Download macOS Catalina, this should eventually open the
App Store at the Catalina page it should say 'Get' press that, if it says 'Open' the App Store has detected a copy of Install macOS Catalina on your mac. If that is the case open your Applications folder and move the installer app to the Trash, empty the Trash.
Make sure your mac is connected to your router via a cabled connection, not WiFi. Restart your mac, go back to the Catalina page in the App Store, and start downloading. Do not use the mac while it is downloading, do not use other devices that share the same internet connection while the mac is downloading.