To generate correctly the reports, your VAT codes table must be up to date, and therefore all standard groups must have a group assigned in the Gr1 column.
To update your table you can use the command Update VAT table. Execute this command by going to the menu Extensions->Netherlands VAT Reports->Update VAT table. When you click on the command a dialogue appears with a list of the rows that will be modified, the group in column Gr1 will be added to these existing rows.
The extension performs the group check on standard codes only, does not work with user-customised ones.
Table already updated
If you run the command to update the VAT table, but the table is already updated, you will be notified by a message, in which case no further changes are necessary.