table and data ctvctyp:
ctvctyp_code    ctvctyp_code_pred
   CO		  Z1
   FE             Z1

FUNCTION get_case_type(case_type VARCHAR2) RETURN VARCHAR2 IS
  cursor  type_cur is
    select ctvctyp_code
    from ctvctyp
    where ctvctyp_code_pred = case_type;       
  temp_type     VARCHAR2(4);
  return_type   VARCHAR2(200);
  counter       NUMBER;
BEGIN
	return_type := '';
	counter := 0;
	srw.message(20,'in case_type is '||case_type);
	open type_cur;
	LOOP
	   FETCH type_cur INTO temp_type;
	   EXIT WHEN type_cur%NOTFOUND;
	   counter := counter+1;		
           if counter = 1 then
		return_type := ''''||temp_type||'''';
	   else
        return_type :=return_type||','''||temp_type||'''';
	   end if;	
	END LOOP;
	
	return return_type;  
END;


In my p_casetype validation trigger:

function P_CaseTypeValidTrigger return boolean is
   tempCaseType   VARCHAR2(200);
   return_type    VARCHAR2(200);
   
begin

    :P_CASETYPE := 'Z1'
    return_type := get_case_type(:P_CASETYPE);  -- I got null returned.
  
  return (TRUE);
end;

But in sqlplus:
declare a bind variable :ctype := 'Z1';
select get_case_type(:ctype) from dual;
returns:   'FE','MI'