error in wcf method

you need to return something outside your if statement:
public  retour ConsultationClientWCF( ref int ID_CLIENT)
        {         
            Service1 ser = new Service1();
            ser.ConsultationClient(ID_CLIENT);
            if ( ser.ConsultationClient(ID_CLIENT).Equals(0)){
                retour rt = new retour();             
                rt.succes = 0;
                rt.msgsucces = "opération réeussite";
                return rt;          
            }
                  
            else if (ser.ConsultationClient(ID_CLIENT).Equals(-1)){            
                retour ht = new retour();
                ht.echec = -1;
                ht.msgechec = "Operation échoué";
              return ht; 
            }
            return null;           
        }