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I'm running macOS 10.15.1 on a macBook Air, 2015 model, and I need to upgrade to the latest version.

There are upgrades available, up to 10.15.5, but when I open "Apple Menu > About this mac > Software Update" I get the following...

Your Mac is up-to-date - macOS Catalina 10.15.1

I know there are newer updates.

I also downloaded and installed the updates from Apple Support, but I always get the following error.

The installation failed. Could not prepare 10.15.3 Update for installation

I have:

  1. Plugged into ethernet
  2. Plugged into power
  3. Restarted in Safe Mode
  4. Flushed the SMC, PRAM, and NVRAM
  5. Tried with and without a VPN
  6. Tried with most of the intermediate DMG
  7. Deleted some files in /var/db/crls
  8. Tried sudo softwareupdate -ia
  9. I've got c. 20Gb of disk space

Any help gratefully received

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2 Answers 2

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Make sure you use the combo updater if you are trying to manually update by more than one "point" release - 10.15.6 is the latest - https://support.apple.com/kb/DL2048

For future reference, Google can always find these very easily - just search "combo update 10.15" etc & the latest will be the first result.

The combo updater is capable of jumping from any point release to the latest.
It may at the same time fix your Software Update control panel issue, though you may not find that out for sure until the next update.

If that still doesn't work, double check your free drive space. Use Disk Utility & the "Free" value rather than the "Available" value.

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    I tried all of those, and yes, I Googled extensively. I was getting the same problem with the Combo's. I my detailed description I referred to them as intermediates, but that included Combo's. I ended up reinstalling, and it's all good now.
    – andrew oxenburgh
    Commented May 23, 2022 at 21:35
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Since you deleted some files in /var - I would just get a good backup of your user files (simply precaution - this doesn’t delete anything intentionally - just it’s easier to back up when the system runs vs if it breaks more).

  1. Boot to Recovery - reinstall the OS entirely

From recovery, the installer will check for firmware updates, T2 or touchbar updates as applicable, then do a full archive of your existing system, drop a pristine and updated macOS on your storage and then migrate things.

On most SSD based macs, the actual time (ex migration script and ex download) is about 5 minutes. So if you don’t have a local caching service or aren’t on fast internet - add 15 minutes for the download. Your customization and amount of apps governs the migration script timing, but it’s almost always faster to get a clean install than pick apart how your mac is broken in my expierience).

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