Persistent macOS Update Failure on MacBook Pro M1 2020

I am using a MacBook Pro M1 (2020). When attempting to update to macOS 26, I consistently encounter the error: "Failed to personalize the software update. Please try again."

Steps already attempted without success:

• Tried multiple networks.
• Created a different user account.
• Booted in Safe Mode.
• Reinstalled macOS from Recovery Mode multiple times.
• Contacted Apple Support multiple times.
• Attempted update without signing into Apple ID and disabled Find My Mac.
• Tried using a bootable USB installer.
• Attempted update via Target Disk Mode from another Mac.
• Tried both Public and Developer versions.
• Currently on macOS 15.7; even upgrading to 15.7.1 shows the same error.

[Edited by Moderator]