How do I update my MacBook Pro Mid-2012?

Your Mac can update to macOS 10.15 Catalina, but not macOS 11 Big Sur.

See here to get Catalina: link-> How to get old versions of macOS - Apple Support

With that said, verify your app compatibility as Catalina cannot run 32 bit apps, and its likely most of your apps are 32 bit and will not work.

Details here: link-> 32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

Back up your data, check your applications for compatibility, and click on El Capitan, High Sierra, Mojave, or Catalina.

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Does the App store open to the correct download page?

Does the error happen when you try to download, or when it tries to open the Catalina page?

Can you try a different internet connection?

If using wi-fi, try ethernet if possible.