Download Sierra Desktop App on Mac with Catalina OS

If you try to launch the Install for Sierra after unzipping the file and it doesn't work, it's probably the result of the Catalina OS which basically blocks anything that doesn’t come through the app store or is certified by Apple. 

Here’s the workaround. These instructions are for installing the files from your desktop. If you have the files elsewhere, you'll have to navigate to that directory in Terminal before running the command.

• Unzip the downloaded file on your desktop
• Open Terminal in the Applications section (get to it in Finder)
• Enter whoami to find out your username
• Run the following command (there is a space in front of users and before and after &&):
• cd /Users/your username/Desktop/sierra_macosx/ && ./installerMacOS.sh
• Once you run the command in Terminal you will likely still get the message: “install” cannot be opened because the developer cannot be verified
• Ignore that and hit enter from your Terminal screen
• Go back to the Install icon and right click on it
• This time you will get the same message only it will also have a warning and ask you if you want to go ahead
• Once you tell it to open it will run the install and you should be able to go through the rest of the Sierra installation steps.

If you are comfortable in Terminal you can also navigate to the directory with the installerMacOS.sh file, and run the sh installerMacOs.sh command and it works the same as above