> when i maintain the Accounting data (Revenu and Expense) at the category level, it works.
> The part of the documentation related to invoice creation does not Mention this error message. Is it okay if i add it in the docu?
It is related to the product (category) configuration. It's not an error. The system is just warning you.
Please note also that the demo database is just a demo. It is not meant to be used in a production environment. Therefore it does not fully comply with the more complex requirements of a production database.
GNU Health is an ERP. Before deploying it in a production environment, master data should be correctly configured. This includes the product and medicament database.