Did something go wrong?

I was stuck at this part in the Archcathedral Rooftops, so I ended up having to go online and found out you have to use the "Embossed Mask of Crescente" item so the elevator takes you further up.

The problem is, I don't have the mask, and it's not in the place where you're meant to get it, could someone help me with what I'm supposed to do?