There are three options for generating Barcodes:
You can activate the Print Rules for your Delivery Methods on the Event Settings page. You can choose to either generate the barcode immediately once the order has been placed or schedule a time for it to be generated.
Sold tickets through a third-party and they are not using the Line-Up barcode? You can import external barcodes for an Event.
You can also generate Barcodes on the individual Order page once it has been completed - but you don't need to do this if number 1 has been done, as then the barcodes will be automatically generated.