1.Press "Command+R" keys while boot.
2.Go to "Utilities" and select "Terminal" to open it.
3.Type "diskutil cs list" to show all of storage volume group.
4.Type "diskutil cs delete UUID" to erase storage volume group which you want.
5.Close "Terminal" and go back "Disk Utility" GUI.
6.Now you can recovery or install your mac os x system.
Reference Link
Can't erase Macintosh HD (FileVault active)
