macOS – Fix ‘Application Is Damaged, Can’t Be Used to Install macOS’ Error on Older Macs

This can be done by downloading the High Sierra installer from the App store and use it to create an installation USB media.

1. Locate and install macOS High Sierra in the Mac App Store.
2. Plug an empty USB flash drive into your Mac with at least 12 GB of space.

3. Run the following command in the terminal:

   sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
   

   Where MyVolume is the name of your USB flash drive.

4. Leave the USB drive plugged in and reboot your Mac while holding the option key (ALT) down.

5. Choose the installer from the boot menu.
6. From the installation environment, choose to install High Sierra on your internal disc. The installation should default to APFS.

A more detailed explanation can be found on the Apple support page: How to create a bootable installer for macOS

Here's what you need to do (example follows High Sierra):

1. Format a USB stick with format Mac OS Extended, scheme GUID Partition Map and name it Untitled
2. Get and run the installer from https://support.apple.com/en-us/HT208969
3. Run through the installer all until you are asked to restart, but then exit the installer
4. sudo mkdir /Applications/Install\ macOS\ High\ Sierra.app/Contents/SharedSupport
5. sudo cp -R /macOS\ Install\ Data/ Applications/Install\ macOS\ High\ Sierra.app/Contents/SharedSupport/
6. sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled

And... if you get an error while installing OSX, e.g. "Installer resource error", then follow these step:

1. ctrl+command+shift+alt+H and open the Terminal
2. cp -rf /Volumes/Install\ macOS\ High\ Sierra/Install\ macOS\ High\ Sierra.app/Contents/SharedSupport/* /Volumes/Untitled/macOS\ install\ data/
3. Reboot!

You're welcome!