2019 MacBook Pro fails to update macOS with 'bridgeOS prepare failed' error

Before you do anything......make sure you have a good backup of everything on that internal SSD.

My first thought is whether any anti-virus software, cleaning/optimizer apps, or third party security software (including VPNs) are installed on this system. These types of apps are notorious for causing all sorts of problems.

You can try booting into Safe Mode to see if that makes any difference, but that may not be enough depending on any system configuration modifications those third party apps may have made which may still be active even if Safe Mode has that third party software itself temporarily turned off. If you have such software installed, then follow the developers' instructions to uninstall them (simply deleting is not usually sufficient). If any were previously installed & later "removed", then many times they can still leave behind remnants or permanently modified system configuration files. It may be worthwhile to run the third party app EtreCheck and post the full report here so we can examine it for possible clues (and confirm successful removal of those apps).

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

To eliminate any network related issues during the update & install process, download the full macOS installer as suggested by the others. Then create a bootable macOS USB installer which will eliminate the need for a good fast network connection....the only network connection that may be needed during the install would be to authenticate or activate the Mac/macOS which is much less of a burden. Although with a 2018-2020 Intel T2 Mac, you will first need to modify the system security settings to allow booting from USB which will require booting into Recovery Mode in order to use the Startup Security Utility.

You can use any other compatible qualifying Mac to actually download the full macOS installer. Generally you will need a model:

• • 2020 to mid-2026 for macOS Tahoe (or a MBPro 16" 2019)
  • 2018 to mid-2025 for macOS Sequoia

Or you may need to perform a DFU Firmware Revive to reset the T2 security chip & system firmware. This requires access to another Mac currently running macOS 15.7+ Sequoia, macOS 26.x Tahoe, or 27.x Golden Gate (with Sequoia you will need to use the Apple Configurator v2.19 app...or perhaps even the recently release v2.21....I would recommend using it with a Tahoe or Golden Gate host too).

It is also possible you may have some sort of hardware issue. The 16" 2019 model is known to have lots of Logic Board issues.

For starters, have you tried updating/upgrading via System Settings > General > Software Update?

Second, if you are using Terminal, in my experience the proper terminal download command is:

softwareupdate --fetch-full-installer --full-installer-version 15.8.1

You would neither use sudo nor specify "macOS Sequoia 15.8.1-24H32"

When attempting to run the install, are you logged into the primary admin account on the Mac (the "Owner" account, that being the 1st admin account created on the Mac)?

Just for clarity, since you have an Intel MBP with a T2 chip ...

Are you trying to update the internal drive ... or an external drive? A T2 blocks installing and maybe updating macOS on an external drive unless you enable it in Startup Security Utility.

48 minutes remaining.

"An error occurred while installing macOS.

You can try again to complete the installation."

if this were an Internet problem,

it would be similar to the reason you get

In Internet Recovery, low negative thousands error messages indicate that your provided Wi-Fi Network is inadequate in some fashion, and cannot be used (for this purpose) in its current state. We have seen -1000 to -6000 reported, often with a letter appended.

Common problems include:

Hidden Network-name

required use of a proxy server to get Internet access

required use of a login page to get Internet access

Use of a login server or certificate to get Internet access

Use of PPPoE in the Mac to get Internet access (typically only applies to DSL)

Use of Fixed IP address rather than good old DHCP

Variance of the time by more than five minutes

Failure of Router to provide workable DNS server addresses, or providing 0.0.0.0 [may produce -2002f]

If you have an Ethernet port, you may in some cases be able to move your Mac close to the Router and connect to the Router using Ethernet.

Hi, thanks for the reply. I’m logged in with my main administrator account, and Secure Token is enabled for it (sysadminctl -secureTokenStatus returns ENABLED). I’m double-checking that it’s the Owner account with diskutil apfs listUsers / and will confirm.

I’ll also try softwareupdate --fetch-full-installer --full-installer-version 15.8.1 without sudo, as you suggested, and report back. If it fails again with BridgeOSSoftwareUpdateError:19 (bridgeOS preflight failure), do you have any other suggestions for a T2 MacBook Pro (MacBookPro16,1)?

I've already tried that, but it's still giving me an error. I also tried manually downloading the installer for the new version and then installing it manually, but it didn't work.

why not the normal:

System Settings > General > Software Update?