Actually, this is an IDE configuration. Visual Studio can't know how to read a sf::Text, or the underlying sf::String(remember that sf::Text is not the text itself, but a graphic representation of a sf::String).
So you need to tell Visual Studio(you can most definitely find some resources by Googling) how it is suppose to read sf::Text for the debugger, and to point it to the sf::String(and of course tell him how to read a sf::String).