This article covers 

Why are we unable to enable the 'I have registered for GST field in the checkout portal' in the checkout and self-serve portal?

Summary 

The billing country needs to be set as mandatory for this to be enabled. Also, we would need to first configure Australia in their list of Taxable countries.

Solution

This field can be enabled only if tax for Australia is set up. You need to configure Australia in the list of Taxable countries. When we checked it seemed that you only have a tax for India configured.

Please refer to the screenshots below :

(this is just an example) You need to just enable this tax so that the field can be used in checkout.

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