SQL & PL/SQL :: Error Message In Execute Immediate Command

I have created a procedure, which should be executed on the below condition with EXECUTE IMMEDIATE COMMAND. But i am getting error.

The error shows the procedure/function name is not existing. But it is exist.

SQL>
1 Declare
2 a varchar2(20);
3 b varchar2(20);
4 c varchar2(1000);
5 begin
6 select to_char(sysdate,'day') into a
7 from dual;
8 select to_char(sysdate,'HH24') into b
9 from dual;
10 if
11 (a='friday' and b>=22)
12 or
13 (a='saturday' and b<=6)
14 or
15 (a='wednesday' and b>=9)
16 then
17 begin
18 EXECUTE IMMEDIATE ('begin'||BACKUP_AUTO_execute_bat_file||'end;');
19 end;
20 else
21 null;
22 end if;
23* end;
SQL> /
EXECUTE IMMEDIATE ('begin'||BACKUP_AUTO_execute_bat_file||'end;');
*
ERROR at line 18:
ORA-06550: line 18, column 32:
PLS-00222: no function with name 'BACKUP_AUTO_EXECUTE_BAT_FILE' exists in this
scope
ORA-06550: line 18, column 4:
PL/SQL: Statement ignored