iATKOS | United Kingdom

Hi there, I wanted to post this as I believe it relates directly to what u/MrMacintoshBlog posted recently about macOS Tahoe being applied to your Mac without your input.

If you happen to see this error, in my opinion it seems like macOS Tahoe has been prepared on your legacy Mac that is using OCLP, whether you wanted it to or not:

The operation couldn't be completed. (com.apple.Buildinfo.preflight.error error 21.)

I've solved this issue happening on my 2012 Mac Pro (but it involved formatting the drive and installing macOS 15.7.2 again):

where I was using macOS 15.7.1, when randomly it stopped booting up all the way and was stuck on a progress bar booting indefinitely (if I let it).

Through troubleshooting, I was able to create a 15.7.1 on a usb flash drive, and when I got to the recoveryOS to reinstall macOS, it said "this volume cannot be downgraded" which was odd to me. In Disk Utility, it also showed it was still a bootable 15.7.1 volume.

During more troubleshooting, I created a 15.7.2 flash drive, and when I got to the point of selecting the drive to install macOS 15.7.2 from the flash drive's recoveryOS, it showed that error that I mentioned above and would not allow installing it.

For the sake of troubleshooting, I had an external SSD also plugged in and running 15.7.1 - that volume, did allow me to apply/install the 15.7.2 update which was great to see.

Eventually, seeing that it did install successfully on my external SSD, I decided to erase my internal Mac Pro storage again, and reinstall 15.7.2 on it - that, worked without any issue. I was not worried about losing data so that was an easy choice for me to try out.

So as of now, the only way I could reasonably fix this problem was formatting the storage, and installing it again. I'm sure there may be other ways with removing snapshots involving updates or something, but I have not been able to test any of that logic yet.

I have a customer of mine with a 2015 iMac with the same issue, and I will help fix his issue later next week when I have time, to further confirm this is the solution.