Locked hard drive
Hard drive locked cannot reinstall OS Sierra on Mac Book Air. Apple cannot verify.
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10.12 Sierra, would reinstall on a Mac that is released on or before September 24, 2016.
It is compatible with the systems on this tip:
https://discussions.apple.com/docs/DOC-10693
If your Mac has been updated to High Sierra, 10.13 or later, and has a solid state drive, the file system may have been changed to APFS, and won't be able to install Sierra or older until you reformat and wipe the drive with HFS Extended GUID format using Disk Utility.
Internet Restore, command-option-shift-R lets you restore the oldest system that came with any Mac that was shipped on or after July 22, 2011.
If you used Filevault to lock your hard drive, please look at this article:
https://support.apple.com/guide/mac-help/encrypt-mac-data-with-filevault-mh11785/mac
If you are thinking High Sierra, Mac OS 10.13 was what you were wanting to reinstall, Macs that shipped with 10.13 also have another level of encryption known as a T-1 chip in some situations.
If you used OpenFirmware password that could be affected by your T-1 system, and locking your system.
https://support.apple.com/en-us/HT204455