No "Buy attributes" on my side, yet I finished the game, is it necessary to make a particular action to unlock this menu?Attributes for existing slave? -> On main menu click on "Buy Attributes" -> Select your Slave -> Click Enter (button on left down corner) -> Select wanted attribute on Attributes list on left or delete unwanted attribute in table on the right -> After that confirm your choices by hitting "Enter" button.
Available attributes for new slaves -> Click in "Investments" -> Select money you want to spent on each segment -> Confirm by hitting "Enter" button.
Remember that leveling segment gives you free shuffle in Solicitation.
Also you need to level Tribute and make Special Request in "Initiate Request" to unlock new places for Solicitation.