Upgrade from El Capitan

Hello

I have a MacBook Pro Mid 2012 with El Capitan installed

When I go to upgrade Mac OS Apple Stores tells me that there are no upgrades available when I am sure that this Mac is compatible with all new versions (Sierra ... till Catalina)

I cannot seem to find a proper DMG file for any original Mac OS

All that is available on the Apple servers are updates which don't work

Who do I solve this problem and upgrade to Catalina