You can only lockpick the chest that are colored red on you map. When locating the chest press
to start the mini game to unlock the chest. Then press
when the indicator is in the blue area then the chest should unlock and collect the items from the chest. Now do that 19 more times around Paris.
Note: You need the Apprentice Locksmith to have access to the chest and i recommend to upgrade your Lockpicking skill to the "Master Locksmith"
to make it easier to unlock the chest.