Enum describing possible invoice errors.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v26/errors/invoice_error.proto)

| Enums ||
|---|---|
| ## `BILLING_SETUP_NOT_APPROVED` | Cannot request invoices for a non approved billing setup. |
| ## `BILLING_SETUP_NOT_ON_MONTHLY_INVOICING` | Cannot request invoices for a billing setup that is not on monthly invoicing. |
| ## `NON_SERVING_CUSTOMER` | Cannot request invoices for a non serving customer. |
| ## `NOT_INVOICED_CUSTOMER` | Cannot request invoices for customer who doesn't receive invoices. |
| ## `UNKNOWN` | The received error code is not known in this version. |
| ## `UNSPECIFIED` | Enum unspecified. |
| ## `YEAR_MONTH_TOO_OLD` | Cannot request invoices issued before 2019-01-01. |