If you see Setup GST & HSN button that means GST% for product on order is not set.
You can click that button and set GST% and HSN for the product shown and again click this button, it will generate an Invoice.
Note:
- In order to keep the system auto create invoices make sure you add GST / HSN on all products.
- If GST% is set and still you see Setup GST & HSN button then it might possible you have deleted a product on your store. In this kind of case please reach us on Live chat and we will assist you.