When I generate a purchase invoice, the cost of the item is reflected in the expenses.
Assuming you are setup using the accrual basis of accounting, that is as it should be.
I would refer you to these guides;
https://www2.manager.io/guides/7189
https://www.manager.io/purchase-invoices
If you are relatively new to accounting, I would also recommend Accounting Basics - Income Statement | AccountingCoach, and focus on “B. Expenses.”
On the chance you intend to enter orders from your suppliers before you are invoiced for goods or services, you should be using Purchase Orders, rather than Purchase Invoices;
https://www.manager.io/purchase-orders
https://www2.manager.io/guides/7210
Lastly, if you intend to only recognize expenses when you pay for them, setup your business to use cash basis and use Payments instead of Purchase Invoices.
https://www2.manager.io/guides/7129
https://www.manager.io/payments