If you have pending orders with bank transfer, you must either cancel the order or complete it in order to create new orders. To check your pending orders, please follow this instruction:
1. Click on Order History in your Shopify:
In this section, navigate to "To Pay" tab:
2. You will see your pending orders here. Click on the "⋮" button:
From here, you can either choose to Continue Payment or Cancel selected order: