This happens if you played on Patch 1.5 and then tried to revert to 1.4. without losing progress. The save schema changed. Your only option is to continue on the newer patch or start fresh.

The update addressed the "Phantom Blade" and stealth mechanics. Prior to 1.4, enemies would sometimes detect Arno through walls or fail to react to dead bodies being dropped right next to them. The patch refined the AI detection algorithms, making stealth gameplay more predictable and fair—a crucial aspect for players attempting "Ghost" runs. Assassin 039-s Creed Unity Patch 1.4 Download

The 1.4 update was designed to solve systemic problems with the game’s engine, specifically focusing on the massive NPC crowds that caused significant lag. This happens if you played on Patch 1

If you are looking for a manual for PC, you will not find an official .exe file. Instead, the patch is integrated into the game’s core version. Your only option is to continue on the