To create GST invoice, or to add GST in invoice,
The product should be added with tax.
If the tax is included in product, then only invoice will be created with tax.
To add product with tax,
Step 1: Please go to Products/Services.
Step 2: Add new Item.
Step 3: Enter product level details, including Tax/GST rate.
Step 4: Click on Add item.
Once the product added with GST, please refer to the below article to create an GST invoice by adding the same product added with GST.
Click on link below to get redirected to the product page,