Hi, I have MacOS High Sierra ( mid 2011) , 2.8 GHz i7,12 GB minne, why can not upgrade to CATALINA

Yes I think so. Your Macs hardware does not meet the requirements for macOS Catalina, if

you want to check read this, How to upgrade to macOS Catalina – Apple Support

How to upgrade to macOS Mojave - Apple Support

Nope,

macOS Catalina - Technical Specifications

General Requirements

OS X 10.9 or later

4GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Catalina:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013 or newer)

https://support.apple.com/kb/SP803?locale=en_US