Download and Upgrade MacOS
Issue
You may have a Mac that does not have a recent version of MacOS or is not compatible with the latest version.
Solutions
From 10.10 to 10.12, you will need to download a DMG file. After that you can download the updater directly from the AppStore using the links below. Once downloaded, run the PKG installer in the DMG file, then run the Install MacOS app that appeared into your applications list.
Yosemite (10.10)
Devices Compatibility List : https://support.apple.com/kb/SP711
Download link : http://updates-http.cdn-apple.com/2019/cert/061-41343-20191023-02465f92-3ab5-4c92-bfe2-b725447a070d/InstallMacOSX.dmg
El Capitan (10.11)
Devices Compatibility List : https://support.apple.com/kb/SP728
Download Link : http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg
Sierra (10.12)
Devices Compatibility List : https://support.apple.com/kb/SP742
Download Link : http://updates-http.cdn-apple.com/2019/cert/061-39476-20191023-48f365f4-0015-4c41-9f44-39d3d2aca067/InstallOS.dmg
High Sierra (10.13)
Devices Compatibility List : https://support.apple.com/kb/SP765
AppStore Download Link : https://apps.apple.com/us/app/macos-high-sierra/id1246284741?mt=12
Mojave (10.14)
Devices Compatibility List : https://support.apple.com/kb/SP777
AppStore Download Link : https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12
Catalina (10.15)
Devices Compatibility List : https://support.apple.com/en-us/HT210222
AppStore Download Link : https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12
DANGER ZONE : Apparently, there is a patch to run Catalina on unsupported Macs. I do not recommend to use it.
Big Sur (11.0)
Device Compatibility list : https://support.apple.com/en-us/HT211238
AppStore Download Link : https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12
Sources
https://www.winytips.com/download-macos-dmg-full-offline-installer/
https://support.apple.com/en-us/HT211683