Pro Tip: If you cannot access this, contact Graphisoft Support directly with your serial number. They can manually grant access to legacy downloads.
If you are truly looking for the original Archicad 12 library because you are working on a legacy project:
Note on Compatibility: You cannot open an Archicad 12 project directly in Archicad 27 without converting it. The process usually requires you to open it in an interim version (like Archicad 19 or 20) first, or use the built-in migration tools, but it is highly recommended to keep a backup of the original file before attempting to upgrade it.
If all legitimate avenues fail, consider these alternatives: