The fix is to make sure you're connected to your Ethernet network, or have your Wi-Fi password on hand. Restart your Mac while holding down the Command + R keys to enter recovery. ... You should now be able to continue the recovery process and reinstall the operating system.

Why is my Mac not letting me reinstall macOS?

First, completely shut down your Mac via the Apple Toolbar. Then, hold down Command, Option, P, and R buttons on your keyboard as you restart your Mac. Continue to keep these buttons held until you hear the Mac startup chime twice. After the second chime, let go of the buttons and let your Mac restart as normal.

What to do when macOS won't install?