Items generally need to be in the backpack to be used outside of the mansion. Check the quest log to make sure you don't have additional requirements.
Viewing post in Yris *Spoiler Free please*
So... I was probably right... just needed to be in my backpack... which honestly is the thing I like least about the game. I think inventory should be as one... rather than that system but i can also see the downside, but no tutorial was ever suggested stating that I needed to move things to my backpack before I figure that out on my own...