SET serveroutput ON;
 declare
 TYPE pointer IS REF CURSOR;
 yo pointer;
 var varchar2(20);
 query_string varchar2(200);
 num number;
 
 begin
 var := 'WORKERS'; --should be the column name!
 query_string := 'select'||var||'from MY_TABLE where MY_TABLE.ID = 2';
 OPEN yo FOR query_string;
 LOOP
 FETCH yo into num;
 EXIT WHEN yo%NOTFOUND;
 dbms_output.put_line(num);
 END LOOP;
 END ;
 /

SQL> declare
  2   TYPE pointer IS REF CURSOR;
  3   yo pointer;
  4   var varchar2(20);
  5   query_string varchar2(200);
  6   num number;
  7
  8   begin
  9   var := 'WORKERS'; --should be the column name!
 10   query_string := 'select'||var||'from MY_TABLE where MY_TABLE.ID = 2';
 11
 12  dbms_output.put_line(query_string);  --<-- here
 13
 14   OPEN yo FOR query_string;
 15   LOOP
 16   FETCH yo into num;
 17   EXIT WHEN yo%NOTFOUND;
 18   dbms_output.put_line(num);
 19   END LOOP;
 20   END ;
 21  /
selectWORKERSfrom MY_TABLE where MY_TABLE.ID = 2
declare
*
ERROR at line 1:
ORA-00900: invalid SQL statement
ORA-06512: at line 14

Error report:
ORA-01722: número inválido
ORA-06512: na linha 12
01722. 00000 -  "invalid number"
*Cause:    
*Action: