pg_upgrade from Postgres 9.5.5 to Postgres 12.3 errors out
The documentation says:

File cloning is only supported on some operating systems and file systems. If it is selected but not supported, the pg_upgrade run will error. At present, it is supported on Linux (kernel 4.5 or later) with Btrfs and XFS (on file systems created with reflink support), and on macOS with APFS.

Obviously you are using different software.
You could use the --link option for a fast upgrade, but that will make the old cluster unusable after the upgrade.
Don't upgrade to 12.3. Use the latest supported minor release, currently 12.5.