Where is the parameter "Group field No. 1" ?

A Views Aggregator table is not a typical views table.
There is no "Grouping field No. 1", as you can apply a grouping function in the column "Group Aggregation" in the table settings.

Every group aggregation function, except Filter rows (by regexp), requires exactly one field to be assigned the Group and compress function. With that done, select any of the other aggregation functions for some or all of the fields. Column aggregation functions may be enabled independently of group aggregation functions.

See the description of the module for an example: https://www.drupal.org/project/views_aggregator

The example in your screenshot can be implemented by views aggregator table by selecting "Group and compress" for the "Auteur" field and "Enumerate" or "Count" for the "Titre" field (all in the Group Aggregation column of the settings). In addition you can apply "Sum" as column aggregation, so the total of all articles is shown.

Hope this helps.