There is no official Naruto Shippuden: Ultimate Ninja 6 for the PlayStation 2. The original series by CyberConnect2 ended on the PS2 with Ultimate Ninja 5 (known in Japan as Narutimate Accel 2) in 2007.
What you are likely looking for is a fan-made modification (mod) of Ultimate Ninja 5. These mods are created by independent teams like the Narutimate Modding Team to add newer characters and features to the classic PS2 engine. Best Sources for the "Ultimate Ninja 6" Mod
Since this is a community project, you can find the ISO files and installation instructions through the developers' social channels: Download Game Ps2 Iso Naruto Shippuden Ultimate Ninja 6
The ISO file is a digital archive of the game disc.
If you’d like, I can:
Which of those would you like?
CDVD > Iso Selector > Browse > Select your Naruto_Ultimate_Ninja_6.iso.System > Boot ISO (Fast).If you see Japanese text, you have a clean ISO. If you see English menus, you have the patched version. There is no official Naruto Shippuden: Ultimate Ninja
If you have decided to move forward, avoid shady "exe download" websites. You want a clean .iso or .bin/.cue file. Here is where the community safely hosts these files (always use an adblocker and a VPN):