FUNCTION vf_compute_field_value ( vendor_form_id_in IN vendor_form.id%TYPE, col_name_in IN VARCHAR2(30) )
		RETURN VARCHAR2 IS
		lv_return_value political_union.goods_declaration_point%TYPE;
		CURSOR c1 IS
			EXECUTE IMMEDIATE
			'SELECT political_union.' || col_name_in || ' INTO lv_return_value FROM '
			|| 'vendor_form INNER JOIN vendor ON vendor_form.vendor_id = vendor.id ' 
			|| 'INNER JOIN address ON vendor.address_id = address.id ' 
			|| 'INNER JOIN country ON address.country_id = country.id ' 
			|| 'INNER JOIN political_union ON country.political_union_id = political_union.id ' 
			|| 'WHERE vendor_form.id = ' || vendor_form_id_in
			;
	BEGIN
				FOR r1 IN c1 LOOP
        EXIT WHEN c1%notfound;
        lv_return_value := r1.':col_name_in';
    END LOOP;
		RETURN lv_return_value;
	END;	

SQL> create or replace FUNCTION vf_compute_field_value ( vendor_form_id_in IN vendor_form.id%TYPE, col_name_in IN VARCHAR2(30) )
  2  RETURN VARCHAR2 IS
  3  lv_return_value political_union.goods_declaration_point%TYPE;
  4  CURSOR c1 IS
  5  EXECUTE IMMEDIATE
  6  'SELECT political_union.' || col_name_in || ' INTO lv_return_value FROM '
  7  || 'vendor_form INNER JOIN vendor ON vendor_form.vendor_id = vendor.id '
  8  || 'INNER JOIN address ON vendor.address_id = address.id '
  9  || 'INNER JOIN country ON address.country_id = country.id '
 10  || 'INNER JOIN political_union ON country.political_union_id = political_union.id '
 11  || 'WHERE vendor_form.id = ' || vendor_form_id_in
 12  ;
 13  BEGIN
 14  FOR r1 IN c1 LOOP
 15  EXIT WHEN c1%notfound;
 16  lv_return_value := r1.':col_name_in';
 17  END LOOP;
 18  RETURN lv_return_value;
 19  END;
 20  /

Warning: Function created with compilation errors.

SQL> sho err
Errors for FUNCTION VF_COMPUTE_FIELD_VALUE:
LINE/COL ERROR
-------- -------------------------------------------------------------------------------------
1/100    PLS-00103: Encountered the symbol "(" when expecting one of the following:
         := . ) , @ % default character
         The symbol ":=" was substituted for "(" to continue.
5/1      PLS-00103: Encountered the symbol "EXECUTE" when expecting one of the following:
         ( select <a SQL statement>

FUNCTION vf_compute_field_value ( vendor_form_id_in IN vendor_form.id%TYPE, col_name_in IN VARCHAR2(30) )
RETURN VARCHAR2 IS

  lv_return_value political_union.goods_declaration_point%TYPE;

BEGIN
  
  EXECUTE IMMEDIATE
      'SELECT political_union.' || col_name_in || ' INTO lv_return_value FROM '
      || 'vendor_form INNER JOIN vendor ON vendor_form.vendor_id = vendor.id ' 
      || 'INNER JOIN address ON vendor.address_id = address.id ' 
      || 'INNER JOIN country ON address.country_id = country.id ' 
      || 'INNER JOIN political_union ON country.political_union_id = political_union.id ' 
      || 'WHERE vendor_form.id = ' || vendor_form_id_in INTO lv_return_value;
      
  RETURN lv_return_value;

END;  

SELECT political_union.* FROM vendor_form INNER JOIN vendor ON vendor_form.vendor_id = vendor.id
...

IF (col_name_in = 'vat_registration') THEN
	RETURN political_union.vat_registration
ElSEIF (col_name_in = 'vat_default') THEN
	RETURN political_union.vat_default
	...