Archicad 12 was released in 2008. Graphisoft no longer hosts this library on their main download portals. Official links are dead, and support tickets for this version are closed. Your only hope is third-party archives or old installation discs.
Cause: Your library path is broken, or you are using a regional library mismatch (e.g., opening a UK project with the US library).
Solution:
Since official direct links are deprecated, you have three options:
