My disk is locked
I was asked to erase the disk and reinstall my OS, however my disk is locked and I can't do anything
MacBook Air 11", macOS 10.12
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You have to boot into Recovery Mode or from a USB installer. Then within Disk Utility you need to select the physical drive to erase. On more recent versions of macOS you must first click on "View" within Disk Utility and select "Show all devices" before the physical drive appears in the left pane of Disk Utility. Once you have erased the physical drive you quit Disk Utility and select the "Reinstall macOS" option.
Make sure to have good verified working backups before erasing the drive or you will lose all your files.