You can’t restore this backup because this Mac does not support booting from HFS

You don’t want to restore. Not here. Restoration restores the original macOS version. Your backup is from a macOS version that is too old to run on this new Mac. Which you know.

Migration Assistant works well in general, but corrupt sources can cause migration issues. With a corruption of the source storage—hardware issues and volume corruptions can cause issues with migrations, and add-on anti-malware and add-on cleaners have occasionally been known to cause corruptions—sometimes migrating from a source storage device directly works, sometimes not.

I suspect you’ve tried this:

Restore your Mac from a backup - Apple Support

And apparently also tried this:

How to move your content to a new Mac - Apple Support

But was that migration attempt after performing at least an update or maybe an upgrade on the source computer?

How to update the software on your Mac - Apple Support

Migration Assistant does work across a range of older macOS versions, so the question then becomes the age of the original Time Machine backup; which macOS version is running on the source Mac?

The other approach I’ve successfully used here—and which can be faster than migrating from a Time Machine backup—is copying the internal storage to external storage, then re-cabling the external storage over to the target system, and using that storage as the source for the migration.

gregherk wrote:

Trying to help my mom set up her new MacBook Pro from a Time Machine back up off her old MacBook. Her old computer is old enough that it could not be upgraded to Catalina, which is the version of MacOS that her new computer is on. When I had to move from El Capitan to Mojave myself, I found the migration assistant didn’t work correctly from my Time Machine backup being on an older os, and I ended up restoring from Time Machine backup, then upgrading. So I was helping her do the same and use the Restore From Time Machine backup option, then upgrade.

However, we got a message, “You can’t restore this backup because this Mac does not support booting from HFS.” Doing a little research, it seems that Time Machine drive should be HFS, and the new MacBooks are AFPS, so I don’t quite understand what’s wrong here and I’m wondering what we need to do to get t working in the future. Shouldn’t you be able to restore from a Time Machine backup?

Set up your new Mac - Apple Support

It is actually very simple. Any mac cannot run a system that is older than it came with (more or less).

A 2020 mac cannot run El Capitan, or even Mojave. So restoring fro Time Machine backup of an El Capitan system will inevitably fail.

If you are migrating there should be no "Restore" from Time Machine.

You have to tell us in detail exactly what you were doing.

There should be no reason not to be able to:

1) connect a Time Machine drive

2) start Migration Assistant

3) choose the drive to migrate from

4) elect to migrate the user account(s) from the drive

I recommend AGAINST migrating applications, settings and other files.

This way you can avoid bringing a lot of cruft, including stuff like old extensions which may not work in the newer OS.