I'm trying to create a function that returns a table but I keep getting this error: ORA-00933: SQL command not properly ended

Question

I'm sorry it's my first time working with functions on SQL. I'm using liveSQL.oracle.com to do homework. But I can't seem to work.

CREATE TABLE Customer_T
    (CustomerID         NUMBER(4)          NOT NULL,
     CustomerName       VARCHAR(25)    ,
     CustomerAddress    VARCHAR(30)    ,
     CustomerCity       VARCHAR(20)    ,              
     CustomerState      CHAR(2)        ,
     CustomerPostalCode VARCHAR(10)    ,

CREATE TABLE Order_T
    (OrderID            NUMBER(5)        NOT NULL,
     CustomerID         NUMBER(4)   ,
     OrderDate          DATE        ,
     FulfillmentDate    DATE        ,
     SalespersonID      NUMBER(4)   ,
     ShipAdrsID         NUMBER(4)   ,

create or replace function GET_HIGHORDER_FUNC(
customemername varchar2, amount int)
return table
as
begin
    select 
    customername, 
    amount 
    from Customer_T c 
    UNION 
    (select customerid, 
     count(customerid) as amount
     from Order_T 
     group by customerid 
     order by amount desc
     fetch first 1 rows only
    ) T
    on c.customerid = T.customerid;
end GET_HIGHORDER_FUNC;

Could someone tell me what I'm doing wrong? Please and thank you!