Redirect Page by function call in before-header process

Hi Sir, I have a question about a function that I call in a before-header process to redirect to the appropriate page.

create or replace function redirect_page(p_user_name in varchar2)

return varchar2

as

v_redirect_url varchar2(100);

branch_page table_1.PAGE%type;

v_state table_1.STATE%type;

v_finalpaper_id table_1.fpid%type;

begin

begin

select fp.state, fp.page,fp.id into v_state, branch_page,v_id

from table_1 fp

left outer join users u on u.uuid=fp.uuid

where lower(u.username)=lower(p_user_name);

if v_state ='REQUESTED'then

v_redirect_url:='f?p=100:95:&APP_SESSION.::NO::P95_ID:'||v_id;

elsif v_state ='APPROVED'then

v_redirect_url:='f?p=100:92:&APP_SESSION.::NO::P92_ID:'||v_id;

end if;

exception

when no_data_found then

v_redirect_url:='f?p=100:1:&APP_SESSION.::NO::';

end;

return v_redirect_url;

end;

Now, when I call the function in the page, I get the 500 Internal Server Error

DECLARE
v_redirect_url VARCHAR2(100);
BEGIN
v_redirect_url:=redirect_page_finalpapers(:session_user_name);
htp.init();
owa_util.redirect_url(v_redirect_url);
apex_application.g_unrecoverable_error := true;
END;

I don't quite understand why this happening and would appreciate some help on here?