You need to configure the purchase configurations before creating a Purchase Order.
The Purchase Configurations are,
The Business Units are budgeted and tracked for the cost, income and allocation.
Add General Ledger Codes that can be associated with the Purchase Order.
Purchase - Additional Fields
Require Additional Fields while creating a Purchase Order ? This configuration provides option to add text, numeric and date/time fields.
Purchase Default Values
Set default values to be populated in the New Purchase Order form.