questions

Hi,
yes, you can restrict some of the categories using Restricted Categories extension https://wpadverts.com/extensions/restricted-categories/

The Categories dropdown you can turn into single select dropdown by using the Custom Fields extension or by adding the code below in your theme functions.php file

add_filter( "adverts_form_load", function( $form ) {
  if( $form['name'] != "advert" ) {
    return $form;
  }
  foreach( $form["field"] as $key => $field ) {
    if( $field["name"] == "advert_category" ) {
        $form["field"][$key]["max_choices"] = 1;
    }
  }
  return $form;
} );

Thank you very much and how can I do so that the main categories are not selected and only the subcategories can be selected? Or is it only possible with the extension mentioned?

It is possible with some programming, but this would require quite a lot of work, i recommend using the Restricted Categories extension to do that.