High Sierra is "not available." Where can I find it?

For more info, my MacBook Pro won't start up. It tries, gets halfway through the progress bar, then locks up. I've tried all the "hold these keys while starting" solutions to no avail. I wanted to try starting it from a bootable drive, thinking that a reload of the OS might fix it. My Mac mini is running Big Sur, and the App Store won't let me download High Sierra to make my bootable drive for the laptop. How can I get around this?

Yes, after downloading for hours it tried to start itself and locked up in the same place.