P. Phillips wrote:
This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.
Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.
Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.
FYI, a "Qualifying Apple Computer" means the Mac being used to create a bootable macOS USB installer must be able to run the OS the user is trying to create for the non-booting Mac (the host Mac does not need to be currently running the OS). A user can use the following article to find which versions of macOS are compatible with various Apple hardware in order to find a Mac capable of creating the bootable USB installer:
https://eshop.macsales.com/guides/Mac_OS_X_Compatibility
Users only need to concentrate on macOS 10.11, 10.13, and 10.15 for the majority of Macs from 2008 to 2019. Just pick one of these three operating systems which is compatible with both the Mac needing an OS and the Mac being used to create the USB installer.
If the user has a 2010+ Mac which has had macOS 10.12.6+ installed at some point in the past, then the user may be able to access the online macOS installer by using Internet Recovery Mode using Command + Option + R.
When asking for assistance in these forums it is best to provide the exact model Mac being used so we can provide more direct & detailed suggestions and answers. Users can get the exact model of their Macs by clicking the Apple menu and selecting "About This Mac", or by entering their system serial number here if the Mac is not able to boot normally (or in Safe Mode):
Check Your Service and Support Coverage - Apple Support
