ORA-00900: invalid SQL statement- when run a package in oracle 12c - oracle

Question

I am using Oracle 12c database and trying to run a package using SQL commands.
CREATE OR REPLACE PACKAGE "PK_CP_OTM" as
FUNCTION F_CP_OPTIMIZATION (
v_current_day IN VARCHAR2,
v_branch_code IN VARCHAR2)
RETURN VARCHAR2;
END PK_CP_OTM;
When I try to execute it using:
DECLARE
BEGIN
EXECUTE IMMEDIATE PK_CP_OTM.F_CP_OPTIMIZATION('20190409','BRNCD001');
END;
It shows:
ORA-00900: invalid SQL statement
ORA-06512: at line 3
00900. 00000 - "invalid SQL statement"
Thanks for your help.

Answer 1

As #Littlefoot said, you don't need dynamic SQL here, you can make a static call; but as you are calling a function you do need somewhere to put the result of the call:
declare
l_result varchar2(30); -- make it a suitable size
begin
l_result := pk_cp_otm.f_cp_optimization('20190409','BRNCD001');
end;
/
In SQL*Plus, SQL Developer and SQLcl you can use the execute client command (which might have caused some confusion) and a bind variable for the result:
var result varchar2(30);
exec :result := pk_cp_otm.f_cp_optimization('20190409','BRNCD001');
print result

Answer 2

There's nothing dynamic here, so - why would you use dynamic SQL at all?
Anyway: if you insist, then you'll have to select the function into something (e.g. a local variable). Here's an example
First, the package:
SQL> set serveroutput on
SQL>
SQL> create or replace package pk_cp_otm
2 as
3 function f_cp_optimization (v_current_day in varchar2,
4 v_branch_code in varchar2)
5 return varchar2;
6 end pk_cp_otm;
7 /
Package created.
SQL> create or replace package body pk_cp_otm
2 as
3 function f_cp_optimization (v_current_day in varchar2,
4 v_branch_code in varchar2)
5 return varchar2
6 is
7 begin
8 return 'Littlefoot';
9 end;
10 end pk_cp_otm;
11 /
Package body created.
How to call the function?
SQL> declare
2 l_result varchar2 (20);
3 begin
4 execute immediate
5 'select pk_cp_otm.f_cp_optimization (''1'', ''2'') from dual'
6 into l_result;
7
8 dbms_output.put_line ('result = ' || l_result);
9 end;
10 /
result = Littlefoot
PL/SQL procedure successfully completed.
SQL>