If error 'com.apple.OSInstallerSetup.error Code=551' occurs when installing macOS
This error can occur when the installer can't make the Internet connections it needs to complete installation. Check your network settings or configuration.
When installing macOS Mojave, you might see an alert saying that an error occurred during installation, followed by ”com.apple.OSInstallerSetup.error Code=551”. Try these solutions:
To find out whether the alert is related to the network settings on your Mac, you can use Network preferences to add a network location. If the default settings in the new network location don't help, you can easily switch back to your current network settings.
If you're using third-party firewall software or other security software, it could be blocking the network access required by the software update. In that case, either reconfigure the software or temporarily disable it until after installing the update.
If your network is using an authenticated proxy (common in some enterprise environments), you might need to check with your network administrator or install the update from a different network. Some authenticated proxies don't allow the specific Internet access required by Apple software updates.
If you still can't install macOS Mojave, contact Apple Support for help.
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