You defined set as
std::set<std::string,CompareLenght> Keyword;
that is the type of Keyword is std::set<std::string,CompareLenght>
But function Get_Word has parameter of type std::set<std::string> *
inline void FindWord::Get_Word(std::set<std::string> *Set_Of_Word)
{ *Set_Of_Word = Parole_Univoche;}
Change it the following way
inline void FindWord::Get_Word(std::set<std::string, CompareLenght> *Set_Of_Word)
{ *Set_Of_Word = Parole_Univoche;}
Also it seems the function should be defined as
inline void FindWord::Get_Word(std::set<std::string, CompareLenght> *Set_Of_Word)
{ Parole_Univoche = *Set_Of_Word; }
because there is no any sense in the assignment
*Set_Of_Word = Parole_Univoche;
Also data member
std::set<std::string> Parole_Univoche;
should be also defined as
std::set<std::string, CompareLenght> Parole_Univoche;