does anyone know why I get back this when creating an invoice with POST to https://xxxxxxxxxx.suitetalk.api.netsuite.com/services/rest/record/v1/invoice ? how do you target discountItem?

{
  "type": "<https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.1>",
  "title": "Bad Request",
  "status": 400,
  "o:errorDetails": [
    {
      "detail": "Unknown field name 'discountItem'. The field does not exist on this record instance.",
      "o:errorPath": "discountItem",
      "o:errorCode": "NONEXISTENT_FIELD"
    }
  ]
}

try creating the record in the UI first if the Discount field is available, usually it's under the "Items" subtab

The discountitem field is not available yet on REST API. If you need to set it then that may be an issue you'll have to find a workaround. If you don't need to set it then just make sure it's not required on the form.