Let's cut to the chase, your save file is bugged out and there is nothing you can do which will fix it, BUT. The only choice you have to get an achievement is to use some else's save file and here I am, saving you from Ubisoft’s greed.

1) Download 'AC3SaveGameTool' --- by Brucie (https://github.com/linuslin0/ACST/releases/tag/v2.4.3)

2) Download my save file (https://drive.google.com/drive/folders/13l6PKknWbGPqjABW_YVcclhQaS584efj?usp=sharing)

3) FInd your saves location C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\savegames

4)
1. Open AC3SaveGameTool
2. Open my saves
3. Select Ac 3 remastered in AC3SaveGameTool
4. Detect for Description id
6. Three dots for Encryption and choose your saves in your saves location, that will decrypt it for you using your saves as an example.


7. Convert!
8. Now backup your saves and replace them with mine.
9. Go offline in ubi connect and launch the game.
10. Do anything to get an autosave and close the game.
11. Now go back online and when asked replace cloud saves with local from your pc.
12. Complete the last Courier mission and you got the trophy

13. Feel free to message me if you need any help!