Open Settings, Invoices and enter the Sales tax rate %. Leave it empty to use your country's standard rate where there is one (10% GST in Australia, 5% GST in Canada). In the US and India enter the rate that applies to your services; in India, for example, 12% for goods transport under forward charge, or 0% when the customer pays GST under reverse charge.
Until a rate is set, the Dashboard reminds you and draft invoices warn that no tax is charged. Invoices with GST are titled "Tax invoice"; in India the GST is shown as CGST and SGST when you and the customer are in the same state, or IGST when you are not, with the place of supply taken from the customer's GSTIN.