Let me guide you on what to key in for the tax code field fo non-GST registered suppliers, support42.
You can select the Out of Scope for the GST/VAT in the tax section when creating a transaction. This way, we categorize the purchase transactions that are out of the scope of GST legislation.
If you need to know more of these tax codes, it's best to contact the IRAS, as well as consulting your accountant to know which tax codes work best for your company.