First, key in your item that on the invoice. As you can see, your items are 0% GST if you had already set your item to SR-0 like in this tutorial How to use the new 0% GST rate in Biztory?
To add 6% GST, just tick the "6%" GST box.
After that, your item will be charged 6% GST.
If the invoice doesn't have GST checkbox in invoice item, you can follow these steps:
Go to your “Icon” and click on “MyBiztory”
Click on “Sales” and go to “Sales Preference”
Tick or untick the box that says “Able to set GST in invoice”
4. Save