If you don't want our software to automatically assign barcodes for you, you can deactivate this option in the Settings.
You can then assign barcodes from the main dashboard to all products with missing barcodes.
How do I assign barcodes to specific products? (Shopify Only)
You can then assign barcodes manually by clicking on the More actions > Assign barcode option available in the Product page, Variant page, Products list page.