In Dragon Quest Builders 2 you will come to a part of the game in the Khrumbul-Dun area that has you opening locks with different colored keys. The key and lock system in Dragon Quest Builders 2 is pretty straight forward, however it’s not like some other games you may be used to.
Keys and Locks come in different colors. Gold keys will open gold locks and silver keys will unlock silver locks. However, simply having the key in your inventory isn’t enough. You actually have to have the key as your selected item in your inventory when trying to open a lock or you will get a notification saying that whatever you’re trying to use is not the right key.
You must move the key into your primary inventory and have it selected to use it in a lock, like in the image above. Doing this you should have no problems in opening locks in Dragon Quest Builders, but unlike other games which will automatically select the correct key for a lock you must do this manually.