Your computer's eligibility must be verified before you can download and restore OS X. When you are asked if you are sure you want to erase the drive, click Erase. How to reinstall macOS if macOS Recovery is unavailable | Macworld.
Give the startup drive a name or use the default name. Be sure to select the volume name and not the device name, which is usually the name of the physical drive - GB WDC WD5, for example. If you never changed its name, the startup volume is listed as Macintosh HD. Select your Mac's startup volume from the list of devices in Disk Utility.
Select Disk Utility from the list of options and click Continue. That's why you made your own bootable media. Use the arrow keys to select the bootable Mountain Lion installer you created earlier and then press the Enter key to start the boot process.
