How can I download Mac OS Sierra and use this command sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Double-click on the .DMG disk image file. This will mount it on the Desktop just like a real disk drive. Double-click on the disk icon to open a Finder window on the content. You will find the installer which you can copy to your Applications folder. To make a bootable USB installer:

How to create a bootable installer for macOS

Bootable USB Installers for OS X Mavericks through Catalina

First, review this introductory article: Create a bootable installer for macOS. Second, read this How To outline for creating a bootable macOS installer. Simply use the Terminal command by copying and pasting the command line for the version of macOS from the list below. You will need an 8GB or larger USB flash drive that has been partitioned and formatted for use with OS X as well as an installer for the desired macOS version that you have previously downloaded from the App Store.

Drive Partition and Format

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the w/type and size info) from the side list.
3. Click on the Erase button in the Disk Utility toolbar. Name the drive, "MyVolume". <---- IMPORTANT!
4. In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button and wait for the Done button to activate. When it does click on it.
6. Quit Disk Utility.

Create Installer

Open the Terminal in the Utilities' folder. Choose the appropriate command line (begins with "sudo") depending upon what OS X installer you want. Paste that entire command line from below at the Terminal's prompt:

Command for macOS Catalina:

sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Command for macOS Mojave:

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

*Command for macOS High Sierra:

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Command for macOS Sierra:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app

Command for El Capitan:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

Command for Yosemite:

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

Command for Mavericks:

sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app

Press RETURN. You will be asked for your admin password. It will not echo to the Terminal window. Then press RETURN again. Wait for the return of the Terminal prompt signifying the process has completed. It takes quite some time to finish. Be patient.

The bootable installer can be used to install the selected version of macOS. Shut the computer down. Insert your flash drive into a USB port. Start the computer like so:

Boot Using OPTION key

1. Restart the computer.
2. Immediately after the chime press and hold down the  "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon for the USB flash drive.
5. Click on the arrow button under the disk icon.

So the version of macOS that you download from Apple downloads as InstallOS.dmg to your downloads folder.

Opening that will give you the InstallOS.pkg.

Double-click on that and it will open an installation window.

This does not install Sierra but creates the Install macOS Sierra.app in your Applications folder.

All good so far, but here we go, when Apple introduced this new method of downloading Sierra

the instructions for creating a USB installer would come up with an error,

Volumes/MyVolume is not a valid mount point

That was six months ago and they have done nothing about it, in fact they have removed the Sierra createinstallmedia

command line from the article, How to create a bootable installer for macOS – Apple Support

it now only shows the commands for Catalina, Mojave, High Sierra and El Capitan.

Apple made this alteration to the article in March 24th 2020.

I have made several requests for Apple to look into this and resolving the error but they have not moved on it.

However recently I found out a way to get round it in another forum.

With the Install macOS Sierra.app in your Applications folder open

Terminal (Applications/Utilities) and enter this text, (use copy and paste)

sudo plutil -replace CFBundleShortVersionString -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist

press Return.

Now enter the createinstallmedia command in Terminal

and press Return.

you should now be able to create the bootable USB installer.