SQL & PL/SQL

When I "exec MAIN('ABC01');" from Editor, I received error "ORA-00900: invalid SQL statement" on the line "EXECUTE IMMEDIATE (ExecProd);" I looked up on the internet. It seems a correct command. What was wrong? Thanks.

PROCEDURE Main(HQ varchar2) AS
ExecProd varchar2(100);
BEGIN
ExecProd := HQ || 'Detail;';
dbms_output.put_line(ExecProd);
EXECUTE IMMEDIATE (ExecProd);
END Main;

PROCEDURE ABC01Detail AS
BEGIN
dbms_output.put_line('Here');
END ABC01Detail;