Firstly, make sure, GST free income is mapped correctly in the extension portal.

Now follow below steps to create a product and select the tax in the invoice.

Step 1: Select No tax – 0.0% in product

 You should select all the tax option so you will be able to select those in invoice when you need.

 Step 2: Select No tax – 0.0% in invoice at line tax/total tax (as per your portal settings, web-service)

 Now your invoice is ready sync without tax.