I am a beginner practicing how to create tables on SQL
Here is what I've done:
Create Table PO (Order_id int primary key, order_date date, customer varchar(1000));
Create table PO_Line_Item (Order_id int not null, Line_number int not null, Product varchar (30), Unit_Price decimal (10,2), Qty decimal (10,2), Tax decimal(10,2));
When I press execute it appeared "there is already an object named 'PO' in the database. What have I done wrong??
If any one have any ideas pls let me know

You can't create a table if a something with the same name already exists. If you have created it in error and it doesn't contain any important data, drop it.

drop table PO;

Now your create table PO ... command should work.