Fix macOS Install Failed Error on Big Sur or Monterey

When does this error show up?

You're trying to install a macOS update—maybe Big Sur 11.6 or Monterey 12.3—and it goes through the whole download bar. Then, right before the installer runs, you get a red box: "Install Failed – An error occurred while installing the selected updates." No details. No retry button that works. It's been happening a lot on Macs with older SSDs or after a previous update left junk behind.

What causes it?

The culprit here is almost always a corrupted download cache. macOS downloads the update files into /Library/Updates and ~/Library/Caches/com.apple.SoftwareUpdate. If a previous download got interrupted (power loss, bad Wi-Fi, or just a wonky Apple server), the leftover files confuse the installer. It tries to verify them, fails, and throws that generic error. Don't bother with disk utility first aid—it rarely helps for this.

The fix: clear the cache and retry

Quit System Preferences if it's open. Don't just close the window—actually quit it (Cmd+Q or right-click > Quit).
Open Terminal (Finder > Applications > Utilities > Terminal).
Run these commands one at a time. They delete the update cache folders. Don't worry, macOS will rebuild them fresh.
```
sudo rm -rf /Library/Updates/*
sudo rm -rf ~/Library/Caches/com.apple.SoftwareUpdate/*
```
You'll be asked for your password. It won't show anything when you type—that's normal.
Restart your Mac. This isn't optional—it clears any leftover locks from the software update daemon.
After restart, go to System Preferences > Software Update. The update should show again. Click "Update Now".
This time, let it finish without touching the Mac. Don't browse or open heavy apps. The download must complete in one go. If your Wi-Fi is flaky, plug in Ethernet.

What if it still fails?

If the same error shows up again, the download file might still be bad. Try a second cache clear—sometimes Apple's servers serve a corrupted file on the first try. I've seen this happen with Big Sur 11.6.3 specifically.

No luck after that? Check your disk space. macOS Big Sur and Monterey need at least 20GB free for a major update, even if the download is smaller. Run About This Mac > Storage to verify. If you're low, move some files to an external drive or use sudo purge in Terminal to free inactive memory.

Still stuck? Try the combo updater from Apple's support website. Search for "macOS Big Sur combo update" or "macOS Monterey combo update"—it's a full installer, not a delta patch. Downloads bigger, but it's more reliable. Run it directly—no Software Update needed.

One more thing

If you're on an older Mac (2013–2016 models), check the date. Monterey and Big Sur have had issues with expired Apple certificates on some updates. A full combo installer bypasses that. I've fixed a dozen MacBook Pros this way. Don't let the size of the download scare you—it works.