Making a Recovery USB
DANGER
Making a Recovery USB will wipe all data on said drive. Back up anything if you need to.
For you to reinstall chromeOS, you will need to make a Recovery USB. There are two methods depending on the Operating System Installed.
Chromebook Recovery Utility (Windows/ChromeOS/macOS)
Download the Chromebook Recovery Utility from the Chrome Web Store.
Launch the app, then follow on-screen instructions. If you are unable to find your device, you can select the "Select a model from a list" option.
Select the USB drive you want to use.
Wait for it to complete.
Manually Flash Recovery Image (Windows/ChromeOS/macOS/Linux)
Go to cros.tech and find your device.
Click on the "Recovery Images" dropdown and select the latest one.
Extract the file.
Windows
Use Rufus to flash the recovery .bin file to USB but make sure to flash in "dd" mode.
macOS/ChromeOS/Linux
Use dd to flash the recovery .bin file to USB.