AI-generated Key Takeaways
- 
          Describes possible errors when requesting invoices. 
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          Errors can occur due to date restrictions, customer status, or billing setup issues. 
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          Specific errors include requesting invoices too old, for non-invoiced or non-serving customers, or for unapproved or non-monthly invoicing billing setups. 
Enum describing possible invoice errors.
| Enums | |
|---|---|
| 
 | Enum unspecified. | 
| 
 | The received error code is not known in this version. | 
| 
 | Cannot request invoices issued before 2019-01-01. | 
| 
 | Cannot request invoices for customer who doesn't receive invoices. | 
| 
 | Cannot request invoices for a non approved billing setup. | 
| 
 | Cannot request invoices for a billing setup that is not on monthly invoicing. | 
| 
 | Cannot request invoices for a non serving customer. |