Hi Bernardo,
You may have to consider the following SAP Note
489676 - External billing documents cannot be cancelled due to VF 188
The below mentioned SAP Notes are also quite relevant to the above Note(Applicable based on the actual issue and the type of issue)
| 198924 | POS upload: Cancellation of the billing document |
| 325365 | Correction of incorrectly posted POS IDocs |
| 857310 | Message VF 056 during the cancellation of a billing document |
PS : Please take some time to read all the relevant Notes and compare it with your current issue and then apply the notes accordingly(As in some case you may not need to remove the entries from Infostructures if they are not active/updating etc)
Hope it helps
Best Regards,
Aram.