As I said in reply #7: "Have you tried Serial.print(device.getName()); ?"
It looks like ".toString()" returns a type 'std::__cxx11::string' and not a type 'String'. Since device.getName() returns a 'char *' you should be able to print that. There are probably other methods of 'BLEAdvertisedDevice' to get other attributes of the device if you want to print more.