com.apple.buildinfo.preflight.error error 21

Well, what happened when you tried that?

Do you have a good backup?

com.apple.buildinfo.preflight.error error… - Apple Community

Quick fixes to 'cannot reinstall macOS in Recovery Mode' (com.apple.BuildInfo.preflight.error error):

1. Restart Mac
2. Check the format of the Macintosh HD
3. Run First Aid to check & repair the Mac hard drive
4. Reinstall macOS in Internet Recovery Mode
5. Reset SMC
6. Reset NVRAM
7. Erase your Mac and then reinstall macOS

https://iboysoft.com/howto/com-apple-buildinfo-preflight-error.html

What does this mean? Com.apple.Buildinfo.… - Apple Community

Run Disk Utility's Disk First Aid from the Restore Mode's Utilities menu, then...

Start up from macOS Recovery

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

In Disk Utility>View, select Show all Devices, highlight the top left entry.

APFS won't work for Sierra or earlier, Sierra or earlier needs MacOS Extended Journaled. High Sierra won't work for APFS on earlier Macs.

What has often worked for others is to use this Restore Method instead of CMD+r keys...

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Messed up Factory Reset on MacBook Pro Mi… - Apple Community

https://discussions.apple.com/thread/254793247?answerId=258936149022#258936149022

https://discussions.apple.com/thread/254872290?answerId=259072534022#259072534022

https://discussions.apple.com/thread/255040953?answerId=259391356022#259391356022