FUNCTION f_get_next_anniversary_date(p_assignment_id    IN  NUMBER,
                                     p_effective_date   IN  DATE,
                                     p_absence_category IN  VARCHAR2,
                                     p_anv_start_date   OUT NOCOPY DATE,
                                     p_message          OUT NOCOPY VARCHAR2,
                                     p_error            OUT NOCOPY VARCHAR2) RETURN DATE
IS
...

declare
v_anv_start_date DATE;
v_message VARCHAR2(100);
v_error VARCHAR2(100);
v_ret_val varchar2(100);
begin
v_ret_val := xxpay_leave_accrual_pk.f_get_next_anniversary_date(201,trunc(sysdate),'AUAL',v_anv_start_date,v_message, v_error);
end;

1. It is tru that A function can have Out poarameter.
   If you are going to use this function in an an SQL quey ,  
   It   wont allow you to use OUT parameter.

2. Though the f_get_next_anniversary_date's return type is DATE 
You are using v_ret_val (The variable you used to collect the 
return value) is of varcahr2 type.  

create or replace TYPE xxoss_r_ann_date AS OBJECT (
assign_id number,
anv_date date,
message varchar2(1000),
m_error varchar2(1000)
)

create or replace type xxoss_r_ann_dates as table of xxoss_r_ann_date

create or replace function xxoss_r_test_object_v2(p_date in date, p_leave_type in varchar2) return xxoss_r_ann_dates
as
v_anv_start_date DATE;
v_message VARCHAR2(1000);
v_error VARCHAR2(1000);
x_ret_val varchar2(100);
xxran xxoss_r_ann_dates := xxoss_r_ann_dates();
begin

for x in (SELECT * FROM per_assignments_f WHERE p_date BETWEEN effective_start_date
          and effective_end_date
          and primary_flag = 'Y'
          and assignment_type = 'E')

loop

x_ret_val := xxpay_leave_accrual_pk.f_get_next_anniversary_date(x.assignment_id,p_date,p_leave_type,v_anv_start_date,v_message, v_error);
xxran.extend;
xxran(xxran.count) := xxoss_r_ann_date(x.assignment_id,v_anv_start_date, v_message, v_error);

end loop;


return xxran;
end;