When creating non-zero percentage tax codes, the field
Account need to have custom balance sheet account selected. Generally, this means you need to create new account in
Chart of Accounts which you can name
GST payable or similar. Then select this account within tax code form.
This is important because default account
Tax payable is only placeholder account. It will correctly report balance on balance sheet. But the account cannot be selected on transactions. This is a problem when tax is being paid. The account cannot be selected as a payment destination.
Please make sure all tax codes are linked to custom balance sheet account of your choice.