Have you read this and relevant pages?
The normal procedure is first a Purchase Order (PO), then a receival note (GRN) and then a Supplier Invoice (menu on right side).
If you do a Direct Supplier Invoice, all these stages will be performed in the background.
If you do a Direct GRN, a PO is done in the background. When the Supplier Invoice is received, you use the Supplier Invoice (menu on right side).
New users are encouraged to setup a training company, to test, verify, etc.
Another variable is what credit terms does the supplier give you?
You can void a PO in Setup -> Void a Transaction
The FrontAccounting Wiki(Manual, examples, tips, setup info, links to accounting sites, etc) https://frontaccounting.com/fawiki/