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
1. In order to keep the system auto create invoices make sure you add GST / HSN on all products.
2. If you have added a new product to your store, set GST for that newly added product. Even if you have set GST% by collection then also you need to click the Update button.
3. If GST% is set and still you see Setup GST & HSN button then it might possible you have deleted a product on your store or you have added a custom product. In this kind of case please reach us on Live chat and we will assist you.