How to get null for rows where data doesn't exist in Postgres while performing joins

I have 2 tables: Items and Rates. Items and Rates are linked using a FK on Rates. If the rate of the item is not present for the day then it is not stored. So for item 5 if no rate is present on 1st January there won't be a record for item 5.

I want item 5 rate as null.

How can I proceed?

Left Join is not working; depending on the data, sometimes I get the date as null, sometimes everything except the item PK is null.

A left join is the right approach, but you need a table that contains all the rows that you want in the result, and you don't have that.
But you can create it with a distinct cross join of your tables.

select to_char(x.date, 'DD-MON-YYYY'), x.pk, rates.rate 
from (
    select distinct date, pk
    from items, rates
) x
left outer join rates
on x.pk = rates.item
and x.date = rates.date
order by x.date, x.pk

produces

With the following setup:

create table items(
  pk integer,
  name varchar(50)
);
    
create table rates(
  item integer,
  date date,
  rate integer
);
    
insert into items (pk, name) values
 	(1, 'A'),
   	(2, 'B'),
   	(3, 'C'),
   	(4, 'D'),
   	(5, 'E');
        
    
insert into rates (item, date, rate) values
   	(1, '1-Jan-2023', 10),
   	(1, '2-Jan-2023', 11),
   	(2, '1-Jan-2023', 12),
   	(3, '2-Jan-2023', 13),
   	(4, '2-Jan-2023', 14);

View on DB Fiddle