Sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia -volume /Volumes/ MyVolume If it has a different name, replace MyVolume in these commands with the name of your volume.
These assume that the installer is in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using.
Open Terminal, which is in the Utilities folder of your Applications folder.Connect the USB flash drive or other volume that you're using for the bootable installer.
