If you are purchasing Non GST item like Petrol or Alcohol for personal use, it will be treated as Non GST item.
1. Create new purchase and choose Purchase Non GST from the dropdown. Fill in the information and save it.
2. In GSTR3B, this amount will be shown in Non GST column
3. In GST2, it will be shown in 2nd section and Eligible ITC would be Ineligible.