Apple no longer allows you to download an OS you can't run, not sure why. :(
macOS High Sierra - Technical Specifications
General Requirements
- OS X 10.8 or later
- 2GB of memory
- 14.3GB of available storage to perform upgrade*
- 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, choose About This Mac. These Mac models are compatible with macOS High Sierra:
- MacBook (Late 2009 or newer)
- MacBook Pro (Mid 2010 or newer)
- MacBook Air (Late 2010 or newer)
- Mac mini (Mid 2010 or newer)
- iMac (Late 2009 or newer)
- Mac Pro (Mid 2010 or newer)
macOS High Sierra - Technical Specifications
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12
macappstores://apps.apple.com/app/macos-high-sierra/id1246284741?mt=12
macOS Mojave 10.14.6 - Technical Specifications
General Requirements
OS X 10.8 or later
2GB 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 Mojave:
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; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)
https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12
MacOS Catalina 10.15.7 - 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)
How to download macOS - Apple Support
https://apps.apple.com/gb/app/macos-catalina/id1466841314?ls=1&mt=12
macappstores://apps.apple.com/app/macos-catalina/id1466841314?ls=1&mt=12
How to upgrade to macOS Catalina – Apple Support
And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...
https://www.stclairsoft.com/Go64/
macOS Big Sur - Technical Specifications
General Requirements
OS X 10.9 or later
4GB of memory
35.5GB available storage on macOS Sierra 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 Big Sur:
MacBook (2015 or later)
MacBook Air (2013 or later)
MacBook Pro (Late 2013 or later)
Mac mini (2014 or later)
iMac (2014 or later)
iMac Pro (2017 or later)
Mac Pro (2013 or later)
Learn about compatibility and how to identify your specific Mac model
https://apps.apple.com/us/app/macos-big-sur/id1526878132?ls=1&mt=12
And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...
https://www.stclairsoft.com/Go64/
Monterey…
macOS Monterey is compatible with these computers - Apple Support
https://wpbeaches.com/apple-minimum-system-requirements-for-macos-monterey-12/
