[1.1.2643] Quest blueprint asks me to build a sarcophagus on top of a wall

[Vincent]

The blueprint is impossible to realise...

I am not sure how this happened, I just accepted a quest.

Save File : https://drive.google.com/file/d/1-bWn7AMf2fDl0amosghbrjAORcvSrua7/view?usp=sharing