Installing El Capiton on an old Mac Book Pro today, I received the error:

OS X could not be installed on your computer

Even if you did a clean install of El Capitan it would not explain why you are seeing OS X 10.6.8. OS X 10.6.3/4 was the original system on your computer. Still, it’s hard to know where the 10.6.8 came from, unless Time Machine has been disabled or disconnected for the last 8 years or so. No packages were eligible for the install. Contact the software amnufacturer for assistance. Quit the installer to restart your computer and try again.' Click to expand. If you're connected to the internet, disconnect it. Re-create the Bootable USB and try installing. Make sure you erase the drive, not the partition.

No packages were eligible for install. Contact the software manufacturer for assistance.

1. May 31, 2021 Another method to fix OS X could not be installed on your computer is to update your macOS in Safe Mode. In Safe Mode, your Mac can run under a pure environment without the interruption of hardware failure and startup items. It can also solve some issues on your Mac. How to enter Safe Mode on Mac?
2. As explained here, OS X 10.11 'El Capitan' fails to install because it is too old (the technical reason being that the signing certificate is no longer valid). But you can trick the OS X installer into thinking that it is 2017 and it will happily run without any errors. How to trick the OS X installer.
3. Then you can try to reinstall macOS or install the updates again. Fix 3: Restore from Time Machine backup. When you macOS stuck in macOS could not be installed on your computer, you can boot your Mac into the Recovery Mode to restore from Time Machine. Here’s how you can do it: 1) Hold down the power button to shut down your Mac.

Quit the installer to restart your computer and try again.

I checked Apple’s support Matrix (it is a very old macbook pro) at https://support.apple.com/kb/SP728?locale=en_GB and it was listed as supported.

After lots of Googling and investigation, I discovered that the date and time were not set to current, the macbook thought it was year 2000.

You can easily reset the time and date by following the steps below:

• Click Utilities on the menu bar
• Select Terminal
• Type date
• Confirm the date is wrong
• Type date 062112422016 (example: June 21st 12:42 2016)
• Exit terminal
• Click Restart

Once the Date and time are configured you will be able to install OS X.

Os X Could Not Be Installed No Packages Were Eligible For Install

Easy fix, for a ridiculous error!