Creating a Void Function in PostgreSQL - postgresql

Question

I am getting an error on this create function code in Postgresql. The error says it is happening around Line 2 at DELETE, but it happens at WITH if I remove that line so I think it is a problem with the format of my Creat Function
create or replace function retention_data(shopId integer) returns void as $$
delete from retention where shop_id = shopId;
WITH ret_grid_step1 as (
select * from (
SELECT
order_id as order_name,
cust_name as cust_name,
email as email,
date(order_date) as created_at,
count(*) as num_items_in_order,
sum(total_price) as sales ,
rank() over (partition BY order_id ORDER BY cust_name ASC) as rnk_shipping_name,
rank() over (partition BY order_id ORDER BY email ASC) as rnk_email
FROM orders
WHERE shop_id = shopId
and order_date is not null and order_date > now()::date - 365 and order_date < now()::date + 1
group by 1,2,3,4
) x
where rnk_shipping_name = 1 and rnk_email = 1
)
insert into retention(shop_id, cust_name, email, last_purchase_dt, total_sales, num_orders, days_since_last_order)
select
shopId as shop_id,
coalesce(b.cust_name,'null') as cust_name,
a.email,
a.last_purchase_dt,
total_sales,
num_orders,
current_date - last_purchase_dt as days_since_last_order
from (
select
email,
max(created_at) as last_purchase_dt,
count(*) as num_orders,
sum(sales) as total_sales
from ret_grid_step1
group by 1
) as a
left join (
select
email,
cust_name,
rank() over (partition BY email ORDER BY created_at DESC) as rnk
from ret_grid_step1
--where cust_name is not null
group by 1,2,created_at
) as b
on a.email = b.email
where b.rnk = 1
and a.email <> '';
$$ language plpgsql;