That's because you need to:

- Mark the variable as a bind by placing a colon before it
- Pass the record variable in the using clause of exec immediate
- Upgrade to 12c!

Do all this and it'll work:

create or replace procedure p ( p hr.employees%rowtype ) as
begin
  dbms_output.put_line ( 'Emp id = ' || p.employee_id );
end p;
/

declare
  rec hr.employees%rowtype;
begin

  rec.employee_id := 10;

  execute immediate 'begin p ( :rec ); end;' using rec;
    
end;
/
Emp id = 10

But the real question is:

Why are you using dynamic SQL?!

You should call it normally:

begin

  demo_pkg.demo_rates(p_rec=>v_emp_rec);

end;
/

If for some reason you really, really need dynamic SQL, you can use:

https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:2664470900346124026