Assassin's Creed (2007) was never released for PlayStation 2. The PS2 lacked the CPU/GPU power for the Anvil engine's crowd AI, climbing system, and open-world streaming.
If you only have a PS2, try these similar-era stealth/action titles:
Recommendation: Play on Xbox Series X|S (best performance) or PC (mods, ultrawide). The PS3 version suffers from screen-tear and 20–30 FPS drops in cities.