Hola, i continue my searches....and found this link https://mahakala.lesc.uec.ac.jp/mahoodle/moodle/docs/33/ja/Main_Page/Managing_a_Moodle_course/Editing_text/Text_editor_FAQ.html
In particular this : "How can I have the Atto toolbar always expanded?"
In Site administration>Plugins>Text editors>Atto HTML editor>Atto toolbar settings, scroll to the Toolbar config box and remove the setting 『collapse = collapse』. Save the changes.
I did not remove (how to remove anyway?) the collapse: i changed the setting i had (1) to 5 (which is the defualt). (see the annex i attached).
And magically the toolbar appeared on the editor.
Best wishes from Asuncion.