Because of the massive size of these patches (often 20GB+), creators split them into smaller parts (e.g., Part 1 through Part 15).
Always copy your original FIFA installation folder before applying a mod. Download FIFA Next Season Mega Patch tashee x00 part12 rar
If WinRAR asks for "Volume 13," it means you are missing the next part in the sequence. Because of the massive size of these patches
Updated promotion/relegation for the Premier League, La Liga, Bundesliga, and more. Step 1: Downloading Part 12 Download FIFA Next Season Mega Patch tashee x00 part12 rar
You cannot "open" Part 12 individually. It is a piece of a puzzle.