I get error in my invoice due to incorrect reference/order number, why is that?  

Receiving service providers may sometimes reject your invoice. This can happen for a few reasons. Firstly, if the reference/order number is incorrectly placed in your invoice, the value is then incorrectly translated and therefore incorrectly placed in your recipients invoice. If you place the reference/order number in the same field as you usually do for other recipients, the problem could be customer specific. That is to say this recipient prefers the value in another place. 

Another reasons why you might get an error message about incorrect reference number, is simply invalid data. Some service providers might have stricter policies when it comes to payment reference numbers, for example banks can reject an invoice if contains a reference number with letters on it.

In both cases, you should contact the recipient and inquire them for a cause of error.