You cannot open Part 12 by itself; WinRAR will prompt you for the location of the next or previous volume if any are missing.
Since these files are usually part of a split archive, you'll need all parts (Part 1, Part 2, etc.) in the same folder to extract them correctly. dietapati.part12.rar
If you get a "Checksum Error" or "Corrupt File" message during extraction, Part 12 might have downloaded incorrectly. Try deleting it and downloading this specific part again. You cannot open Part 12 by itself; WinRAR